For PVD coating, high-purity, solid metals are used as layer materials. Depending on the desired properties of the coating, these metals may be, for example, titanium, aluminium, or chromium, as well as zirconium and silicon. This material is referred to as the target.

The composition, thickness and properties of the coating can be controlled by the selection of the target, the process parameters, and the deposition conditions. This allows regulation of such factors as the structure and hardness of the workpiece, and also its thermal resistance.

The desired coating thickness furthermore depends on the size and purpose of the workpiece. In principle, the coating can be up to 10 μm thick. For micro tools, on the other hand, the coating thickness is usually less than 1 μm.

There are different PVD coating methods, and these can also be combined. Those most commonly used are:

• Arc-PVD: During arc evaporation, an arc is created between an electrode and the coating material in order to detach particles from the target.
• Sputtering: The target is bombarded with magnetically deflected ions or electrons
• Laser: Laser beams are fired at the material in order to initiate evaporation.

At Härtha, we offer sputtering and arc coating. In principle, the different techniques all follow the same sequence of steps.

Evaporation

During evaporation deposition, the target is heated so much that the atoms on the surface are released as gas, and thus become available for the next step. Different techniques can be used to accomplish this. At Härtha, we use the arc technique.

To ensure controlled conditions and prevent interaction with air molecules, evaporation takes place in a vacuum.

Reaction

To deposit the evaporated material onto the workpiece surface, a reactive gas is now supplied, and this combines with the metal vapours. The choice of gas has an important influence on the properties of the coating. Generally, the gas of choice is either a gas containing carbon or nitrogen. These gases deliver strong adhesion, and form nitride and oxide compounds that protect against rust and corrosion.

In order to prevent unwanted chemical reactions, this step takes place in a chemically non-reactive atmosphere. This can be achieved by using an inert gas such as argon. To ensure that the coating thickness is uniform across the entire workpiece, the workpiece is rotated about multiple axes during this step.

Deposition

 
In the final step, the evaporated atoms of the target condense on the workpiece surface, forming a thin film coating.

Wear-protection coatings at a glance

PVD coatings are suitable to function as wear-protection coatings. Frequently used basic types include titanium nitride, titanium carbonitride, and titanium aluminium nitride.

An overview of coating systems and their properties can be found in our table.

PVD coatings are used across a multitude of industries and for a wide variety of components:

• Cutting tools
• Forming and shaping tools
• Plastic moulds
• Industrial components
• Automotive components
• Jewellery and watches
• Medical engineering
• Decorative and sporting applications
• Aluminium die casting

PVD coating is a surface treatment. To prevent changes to the microstructure and hardness, and to ensure dimensional stability, the material must be subjected to a heat treatment appropriate to the coating.

 

Since PVD coatings can be applied below 500 °C, the process is highly suitable for high-speed steels, hot worked steels, and some cold worked steels.

 

Even steels tempered at very low temperatures are generally suitable for coating – with special coating systems for low-temperature processes (between 250 °C and 450 °C).

The key advantages of PVD coatings at a glance:

• Great dimensional stability thanks to low coating thickness
• High adhesive strength
• Increase in wear resistance and hardness
• Reduced friction, thanks to smooth surfaces
• Coating temperature up to 450 °C
• Any type of layer structure (mono-layer, multi-layer)
• Visual refinement

At Härtha, we offer you the PVD coating and DLC coating processes. We coat workpieces of different sizes, from the micro range to a diameter of 500 mm. In addition to standardised coatings, we also develop tailor-made solutions that will fit your specific application.

We inspect all PVD coatings visually. If you require an in-depth test, we can recommend non-destructive testing methods.

Before we can quote you an offer for PVD coating or find a different coating solution for you, we need you to provide us with the following information first:

• Purpose of application
• Material designation
• Thermal pre-treatments
• Desired coating thickness in µm

Find a location near you. Our location overview shows you which Härtha locations offer PVD coating and which other metalworking processes we provide.

DLC coatings are diamond-like carbon coatings. There are various techniques available. The most common are PVD coating, a physical process, and plasma-assisted chemical deposition or Plasma-Assisted Chemical Vapour Deposition, "PACVD" for short.

 

The techniques are further categorised into hydrogen-containing, amorphous carbon coatings (Amorphous Diamond-Like Carbon, "ADLC" for short) and hydrogen-free coatings.

 
 

Hydrogenated amorphous carbon coating

 

ADLC coating is the most common variant of DLC coating. It is usually applied using the PACVD technique. These DLC coatings achieve hardness levels between 1,200 HV and 2,200 HV and dry friction values of 0.1 to 0.2 against metal. They can generally be applied at temperatures up to 300 °C.

 

The addition of doping elements makes it possible to adapt the properties of the coating to the desired outcomes.

 

 

Hydrogen-free DLC coatings

 

Hydrogen-free DLC coatings are usually harder than ADLC coatings, and have a very low coefficient of friction as well. They are usually applied to the workpiece by means of the arc-PVD technique. This procedure produces tetrahedral amorphous carbon (ta-C), which offers a higher abrasive wear resistance than amorphous carbon.

 

These particularly wear-resistant DLC coatings are often used in the industrial sector, for example for automotive components, racing vehicles, and industrial components, as well as for oil and petrol applications, camshafts, valves, hydraulic pumps, and much more.

 

 

PVD vs. PACVD processes

 

Coating process	PVD	PACVD
DLC coating	HC08  Cr+a-C:H:W	HC09  Cr+a-C:H:W+a-C:H
Hardness HV 0.05	1.200 – 1.800	2.200 – 2.000
Application temperature	350 °C	300 °C
Deposition temperature	< 200 °C	< 200 °C
Colour	Grey	Black
Coating thickness µm	1-6	1-6
Friction coefficient	0.2 dry, against steel	0.1 dry, against steel

The DLC coating takes place at very low temperatures (below 200 °C). The process is therefore equally suitable for refining aluminium, copper, brass or low-tempered steels. Some suitable materials at a glance:

• Chrome
• Steel
• Aluminium
• Copper
• Titanium
• Brass
• Molybdenum
• Silicon

DLC coatings are outstandingly suitable for protecting components that are exposed to high loads and extreme friction. They are used in fields such as the automotive industry, mechanical engineering, and the food industry. Typical applications are:

• Hydraulic drives
• Fuel injection systems
• Pumps
• Cutting blades
• Filling plants

 

The low friction coefficient and the great hardness of DLC coatings protect the component against heavy wear, and thus against pitting, chafing and seizing. 

• Low coating temperature (up to 200 °C)
• Very thin layer thickness (usually 1 μm – 6 µm)
• Maximum application temperature of up to 350 °
• High adhesive strength
• Great dimensional stability
• Microhardness up to 2,200 HV
• High chemical resistance
• Low coefficient of friction (0.08 – 0.1)
• Low adhesion tendency
• Corrosion protection
• Visual refinement

Depending on your specific application, various coating systems may prove to be ideal for your workpieces. In addition to standardised PVD coatings, we also offer DLC coatings that are specially tailored to your needs.

HÄRTHA operates various locations in Germany, Italy, and the Netherlands. Refer to our location overview to learn where we perform DLC coating, and which other processes we offer in your vicinity.

Before we can quote you an offer for DLC coating, we require as a first step the material designation of the workpiece to be treated and, if applicable, information on any thermal pre-treatments.

 
The objective of phosphating is to form a phosphate layer on the metal surface. The process begins with pickling the component to remove the natural oxide layer from the metal. This step leads to cations going into solution, with the evolution of hydrogen.
 
The component is afterwards immersed in a phosphate solution to precipitate barely soluble phosphates. Depending on the composition of the solution, an iron, manganese or zinc phosphate layer now forms. The process can be repeated if necessary. Phosphating is sub-categorised into layer-forming and non-layer-forming phosphating.  
 

Non-layer-forming phosphating

Non-layer-forming phosphating is designated as such because no metal cations pass from the phosphate solution into the treated metal. Instead, all cations which form the layer originate from the material itself. Although providing great dimensional stability, this process provides less corrosion protection compared with layer-forming phosphating.

Layer-forming phosphating

 
In this variant of phosphating, cations from the base metal may also be involved in the layer formation, but the majority of the positive metal ions originate from the phosphate solution. For fitted components, it is necessary to take account of the additional layer, which can range in thickness between hundreds of nanometres to a few micrometres.

Metals and types

 

Another distinguishing feature of phosphating is the metal that contributes to forming the layer structure – the most common are iron, zinc or manganese. Depending on the desired properties, two or three of these metals can be applied at the same time.

 

Iron

Iron phosphating is used to provide corrosion protection and to form the basis for the painting of components such as metal sheets. It is regarded as the simplest and cheapest option, because surface activation and phosphating take place in the same work step. Iron phosphating requires temperatures from 25 °C to 65 °C and a pH value between 4 and 6. Iron phosphate layers weigh between 0.2 and 0.8 g/m². Iron phosphating is suitable for workpieces made from iron, aluminium, or zinc.

 

Zinc

For zinc phosphating, the metal surface must be pre-treated in order that a fine-crystalline zinc phosphate layer can be achieved. For this purpose, the process begins with a titanium salt bath. Nickel is added to the subsequent bath at 35 °C to 80 °C and pH values range from 2.2 to 3.2. This process causes pores to form on the metal surface, which absorb oils particularly well and thus increase corrosion protection. It also results in improved paint adhesion on blank and galvanised steels. The zinc phosphate layer has a matt appearance, and a light grey to medium grey colour. It weighs 1.5 to 30 g/m².

 

Manganese

Manganese phosphating takes place at temperatures from 90 °C to 95 °C and at a pH value between 2.2 and 2.4. Manganese phosphate layers weigh between 10 and 25 g/m². They provide friction-reducing properties, good corrosion protection, and high absorption capacity for oil – ideal for use in transmissions or slide bearings. Since it produces a dark grey to black colour combined with a silky-matt look, manganese phosphating is also used for the visual enhancement of small arms.

 

INFO: Coating thickness
The composition of the phosphate solution determines the thickness of the conversion layer.While phosphating with manganese or zinc can produce layers having a thickness of up to 2 μm, iron phosphating layers are usually only a few hundred nanometres thick.

 

 

Other metals and types

Trication phosphating is a phosphating process that involves zinc, nickel, and manganese. Other additives frequently used for phosphating are sodium nitrate, sodium nitrite, sodium fluoride, and calcium. Less common is the use of titanium, zirconium, and copper sulphate. Nickel is largely avoided because it is dangerous to health.

 

Features and advantages

• Adhesion promoter: the phosphate layer is firmly anchored in the base metal
• High level of corrosion protection: Pores and capillaries ensure a high capacity for absorbing oils and paints that provide corrosion protection
• High lubricity: Reduction of friction and wear
• Appearance can be adjusted via the process: fine crystalline to coarse crystalline structure, light grey to black
• Insulation: Phosphate layers offer high insulation resistance
• Rust resistance: damaged layers are nearly insusceptible to rust

 

 

While commonly applied to steel, phosphating is often used for aluminium as well as galvanised and cadmium-plated steels. It is primarily used as a preparation for coating. Since the phosphate layer adheres very well to metallic surfaces, and its pores or capillaries form the perfect basis for oils, greases, and paints, it is used as an adhesion promoter.

 

Phosphating as corrosion protection

The phosphate layer itself already offers corrosion protection; this can be further improved, e.g. by waxing or oiling. In addition, the layer is rust-resistant and can largely prevent underlying corrosion. Zinc phosphating delivers greater resistance to corrosion than iron phosphating.

 

Improved anti-friction properties because of phosphate layers

Phosphating improves the anti-friction properties of workpieces. Zinc phosphate layers in particular are used for the cold forming of steel. They react with alkaline soaps to form zinc soaps, which withstand higher temperatures and form an ideal lubricant system.

 

Manganese phosphate layers are also frequently used to prevent workpieces from seizing up because of insufficient lubricant. They can reduce wear on the friction surfaces.

 

Electrical properties

Phosphate layers are ideal for insulation, as they offer high electrical resistance at a small thickness. They are used, for example, in electrical sheets for magnetic cores.

 

Phosphate layers offer greater corrosion protection than burnished layers, because their irregular contour provides better adhesion for anti-corrosion oils and greases than the amorphous surfaces created by burnishing.

 

Burnishing is particularly well-suited for precision parts, due to its great dimensional stability because the layer is formed within the component surface. Conversely, layer-forming phosphating requires the thickness of the layer created on the surface to be taken into account.

 

The success of phosphating hinges on the exact coordination of temperatures and reaction times, as well as of chemicals and their concentration. Thanks to our long years of practical experience in bath management and advanced systems, we can guarantee the exact regulation of these parameters. We ensure that your base material is not damaged, and we adjust the thickness of the phosphating layer, the crystal sizes, and the surface adhesion to the specific purpose of your workpieces. To benefit from maximum quality, rely on an expert partner.

 

 

Maximum workpiece size:
1,800 x 320 x 500 mm

 

 

We are delighted that you have decided to have your workpieces phosphated at Härtha. As a first step, please provide us with the following information:

• Material designation
• If applicable, information on any previous heat treatments
• If applicable, information on the desired layer thickness (in µm)

 

 

Refer to our location overview to find a location near you where we offer phosphating and many additional processes.

 

Anodising is divided into three phases - pre-treatment, anodic oxidation, and sealing. The use of a dye bath is a further optional step, and is performed before sealing, when dyeing of the workpiece is desired.

Pre-treatment

So that the workpiece surface is optimally prepared for oxidation, it is first thoroughly cleaned and freed of dust and grease. In addition, thermal pre-treatments such as hot bending or welding can cause changes in the material that carry over to the anodised coating. Chemical pickling can even out these irregularities and achieve the most homogeneous microstructure possible. DIN 17611 defines all methods of pre-treatment for anodising.

Anodic oxidation

The actual electrolytic oxidation process, carried out in an electrolyte bath, now follows. In this step, the aluminium workpiece is immersed in oxalic or sulphuric acid. At the same time, an anodic circuit is created by a connection with the positive terminal of a DC source. In addition, a cathode is used for this step. The electrodes used are usually plates made of titanium or lead, because they are passive to the electrolyte. Electrolysis now splits the water into hydrogen and oxygen. The hydrogen escapes at the cathode, and the oxygen reacts with the aluminium of the anode.

 

A thin barrier layer is formed, continually growing into the aluminium. Capillary-like pores develop.In the end, about a third of the anodised coating protrudes beyond the original surface level of the workpiece.

Dyeing

Once all residual acid has been removed, the workpiece can be dipped in a dye bath. The open pores absorb the dye, and so it is well protected against abrasion. This process may take up to 20 minutes. Common colours are gold, purple, blue, red and green, as well as black. It is not possible to colour the material white.

Sealing

In the final step , the workpiece is boiled in demineralised water. This forms a transparent alumina hydrate, which closes the remaining pores on the surface of the anodised coating.

 

The DIN 8580 standard classifies anodising as a coating process. More specifically, it is a galvanic coating applied by means of electrolytic oxidation of aluminium.

 

Aluminium naturally reacts with the oxygen in the air to form aluminium oxide. This reaction creates a transparent oxide layer on the surface which is a few nanometres thick and protects against corrosion. Hardness and wear resistance remain unaffected.

 

Electrolytic oxidation is used to enhance this layer formation, making it possible to achieve coating thicknesses of 5 μm to 25 μm. As long as the workpiece surface remains unharmed, this coating reliably protects the workpiece from corrosion.

 

The hardness levels that can be achieved with anodising range between 200 HV and 400 HV. The hard electrolytic oxidation process produces hardness levels up to 600 HV. In addition, the anodised coating can be dyed for decorative purposes.

 

INFO: What metals can be anodised?
Anodising is possible only with aluminium. This is indicated by the acronym "Eloxal", which stands for "electrolytic oxidation of aluminium" and is often used as a synonym. In the case of ferrous metals, oxidation will result in rusting rather than in a refinement of the surface. For such metals, other types of surface refinement are used. One example is the burnishing of steel.

 

Anodising provides a number of key advantages for aluminium components:

 

• High corrosion resistance for outdoor use
• Scratch-resistant, hard surface without peeling
• Decorative properties
• Low coating thickness
• Electrical insulation

 

The main purpose of the electrolytic oxidation process is the corrosion protection of aluminium semi-finished products, such as sheets, rods, pipes, and profiles. However, the dyeing process is also frequently applied for decorative purposes, for example for fasteners such as snap hooks.

 

INFO: Additives to alloy components
The basic prerequisite for successful electrolytic oxidation is a material having electrolytic oxidation quality, "EQ" for short. Conversely, anodising may lead to structural changes or staining if used on cast aluminium or similar alloys. Examples of materials commonly subjected to anodising are AlMg 1 hh EQ or AlMgSi 0.5 EQ.

 

For problem-free coating formation during anodising, no foreign metals must be trapped in the aluminium, and any roughness must have been removed from the aluminium surface.

 

The properties of the anodised coating depend on the temperature, current density, and voltage, and also on the composition of the electrolyte. While the base material of the anodised coating is always aluminium oxide, adding chemical additives to the electrolyte can improve properties such as durability.

 

Another factor playing a major role is the use of direct current or alternating current. The coating thickness is essentially influenced by the duration of the treatment.

 

Unlike aluminium, aluminium oxide does not conduct electricity. The oxide layer thus affects the insulation capability of the workpiece.

 

Anodised aluminium rarely requires any maintenance. If used in a normal environment, the material needs to be cleaned only once per year - in rough conditions, twice per year. To preserve the surface, it is advisable to use pH-neutral cleaning agents.

 

 

While powder coating conceals the surface structure of a workpiece, this remains largely discernible after anodising, because the anodised coating is only a few nanometres thick. In addition, the coating is matt.

 

 

1,600 x 600 x 900 mm

 

 

The Härtha hardening plant operates subsidiaries spread throughout Germany and Europe. Refer to our location overview to learn at which locations we perform anodising.

 

 

We will be glad to send you a non-binding proposal for our anodising services. Before we can do so, we need the corresponding material designation. We will then send you our response by return, with additional information.

 

 
Electroless nickel plating is a dimensionally stable coating method for nearly all types of metal. A chemical process not requiring an external current deposits the electroless nickel layer at a temperature of approx. 90 °C. The result is an even distribution of the layer thickness across the entire workpiece.
 
Because electroless nickel plating is exceedingly time-consuming, the process is very expensive. It takes about one hour to produce a thickness of 10 µm. Consequently, thicker layers exceeding 50 µm are rare in practice.
 
INFO: Difference from galvanic nickel plating
Electrons are necessary to separate nickel ions from the metal. During galvanic nickel plating, an electric current is supplied from an external source such as a rectifier. In contrast, electroless nickel plating produces the necessary electrons itself through a redox reaction that takes place in the bath. This allows contour-accurate coatings with a tolerance between ± 2 μm and ± 3 μm at a coating thickness of 8 μm to 80 μm. However, stresses may form in the electroless nickel coating at thicknesses of 50 µm or more.
 

Properties of the coating

 

The properties of the nickel-phosphorus coating can be specifically controlled via the phosphorus concentration. Concentrations of 3% to 7% are classified as a low phosphorus content, 6% to 9% signify a medium phosphorus content, and 10% to 12% indicate a high phosphorus content.

• The higher the phosphorus content, the greater the protection against corrosion. It is furthermore important that the coating is free of pores. This depends on the type of material and the pre-processing of the workpiece (milling, polishing, etc.), as they affect adhesive strength. The corrosion protection layer is usually at least 30 μm to 50 μm thick.
• The lower the phosphorus content, the greater the increase in hardness or wear resistance. An additional heat treatment (up to one hour at a maximum of 400 °C) can produce a hardness level of 800 to 1,100 HV. Common coating thicknesses range between 10 µm and 50 µm.

The appearance of the electroless nickel coating can be altered only to a limited degree, for example by means of brighteners in the electrolyte. Adjustable properties such as the density of the grain boundary can affect the appearance only slightly. The appearance thus largely depends on the pre-processing of the workpiece – shiny surfaces remain shiny, matt surfaces remain matt.

The adhesion strength is also dependent on the type of material and on previous treatments. Heat treatment at low temperatures and a long holding time improve adhesion strength.

Types of processes

 

Mid-phos electroless nickel plating
(medium phosphorus content)

Mid-phos electroless nickel plating produces a medium phosphorus content (6% to 9%). It is used to increase the hardness (approx. 600 HV) and wear resistance of workpieces and to achieve good corrosion protection (over 480 hours at s/min = 0.030 mm). The workpieces become ferromagnetic. A heat treatment can increase hardness to approx 1,000 HV.

High-phos electroless nickel plating
(high phosphorus content)

High-phos electroless nickel plating produces a high phosphorus content (10% to 12%). This process is ideal for workpieces that are intended to possess very high corrosion resistance (over 500 hours at s/min = 0.030 mm) and must not be magnetic. Wear resistance and hardness (approx. 550 HV) are lower compared to the mid-phos process. Here too a thermal post-treatment can lead to greater hardness (approx. 900 HV).

Thermal treatment

A thermal post-treatment of electroless nickel-plated workpieces is primarily used to increase the hardness as far as 1,000 (± 50) HV; and also to improve wear resistance and coating adhesion. For this purpose, the workpieces are tempered at 230 °C to 400 °C.

Hybrid coating

Hybrid coating combines all the advantages of the electroless nickel and hard chrome plating processes . Electroless nickel plating is followed by hard chrome plating. This procedure creates a coating system that offers excellent corrosion protection and very high wear resistance. The actual coating thickness depends on the purpose of the application. Workpieces with a hybrid coating are suitable for use in harsh environments marked by high chemical and mechanical loads.

Barrel coating

During a barrel coating procedure, the workpieces are placed in a perforated barrel that rotates around its own axis. The rotation ensures that all workpieces are treated in the same way. This process is used for bulk goods such as screw connectors.

Rack coating

Electroless nickel plating by means of a rack is suitable for large and bulky workpieces or for precision components. The components are hung up or fitted on a rack and guided carefully through the process.

Sealing

Sealing provides protection to porous component surfaces. It is suitable, for example, for castings. A transparent organic protective film increases corrosion resistance, thus reducing the workpiece’s sensitivity to fingerprints and also improving its appearance.

Electroless nickel plating offers a number of important advantages for the treated components:

 

• High corrosion protection as well as great hardness and wear resistance
• Hardness and wear resistance can be further improved through a heat treatment
• Desired properties can be precisely controlled by means of the phosphorus content (e.g. hardness, ductility, magnetic properties, etc.)
• Uniform coating thickness even for workpieces with a complex geometry
• High reproducibility and series production reliability
• Excellent solderability
• Layer build-up without lead and cadmium
• No need for an external power source

 

 

Its extensive application possibilities and wide range of treatable metal types make electroless nickel plating indispensable in many industries:

 

• Toolmaking
• Mechanical and plant engineering (e.g. robotics)
• Automotive and aviation (e.g. drive & control technology)
• Electrical industry & microelectronics (e.g. heat sinks, connectors, batteries, and components to be soldered)
• Medical engineering

 

 

In principle, all metallic base materials are suitable for electroless nickel plating. In addition to steel and stainless steel, these materials include aluminium, zinc die-cast, and non-ferrous metals such as copper, brass and bronze, as well as other materials based on sample coatings.

 

 

 

During electroless nickel plating, nickel ions are transferred from the bath into the workpiece. A stable nickel content and the appropriate pH value are ensured by top-up pumps and regular checks. The age of the bath is indicated by the metal turnover (MTO). A constant temperature of 90 °C must be maintained. After one to two weeks, it is necessary to prepare a completely new bath.

 

This elaborate bath management paired with the long process duration makes electroless nickel plating costly, with more complex equipment technology than that used for galvanic processes.

 

 

 

Besides chemical nickel plating and chemical nickel coating, which both indicate the chemical nature of the process, the process is known by a number of other names, such as electroless nickel ("EN" for short). With regard to the elements contained, the terms nickel-phosphorus alloy or, for short, "nickel phosphorus" or "NiP", have become established. Other distinguishing terms are low-phosphorus/Low-Phos, medium-phosphorus/Mid-Phos, and nickel high-phosphorus/High-Phos, which are indicative of the phosphorus content.

 

 

The dimensions of our system are:

Length: 2,100 mm
Width: 1,150 mm
Height: 730 mm

 

We can process barrel goods Special dimensions upon request

 

At Härtha, we offer electroless nickel plating at various locations. Learn more in our location overview.

 

 

Would you like us to treat your components with electroless nickel plating? Please start by providing us with the material designation and information about thermal pre-treatments (if applicable). We will then get back to you.

 

 
The DIN 50983 standard defines burnishing as a manufacturing process that converts the metal surface, i.e. no coating. For this purpose, the component is immersed in an alkaline or acidic solution, and then heated. 
 
A chemical reaction between the burnishing agent and the material leads to the formation of a so-called patina – a black layer of mixed oxide on the surface of the component. This layer is only about 1 μm thick and has no impact on dimensional stability. 
 
The burnishing of metals requires both a pre-treatment and a post-treatment. The following process has become established:

 

Pre-treatment

In order to successfully burnish steel, the workpiece must first be freed from residues and impurities, and then cleaned thoroughly. The following steps are necessary to accomplish this: Degreasing, rinsing, pickling, and rinsing again

Burnishing

Burnishing salt is stirred into cold water to prepare the burnishing bath. When combined with a heat supply, exothermic processes cause the temperature to rise to the boiling point.

Next is the actual process of burnishing. The component is now immersed in the burnishing salt solution with the help of barrels, racks, or sieves until the surface turns deep black. Throughout the entire process, the burnishing bath is kept at the boiling temperature. This process may take between five and thirty minutes. Its duration depends on the geometry of the component, its type of material, and its intended use.

Post-treatment

In order to remove any salt residue left on the metal after burnishing, the workpiece is rinsed again, usually with hot water and ultrasound. After that, corrosion protection is applied to the surface.
 
INFO: Durability and protective effectiveness of burnishing
Preservative oils or greases are used to protect the burnished layer against corrosion. These agents allow it to withstand mechanical stresses such as bending and pressing, and make it resistant to heat up to 300 °C. Even stresses from the alkaline milieu, i.e. conventional lubricants, varnishes, and solvents, will not affect the treated layer. Burnished workpieces are therefore well suited for use indoors. Acids, on the other hand, dissolve the burnished layer. For this reason, treated components intended for outdoors require additional protection.

 

Burnishing as a surface treatment offers a number of advantages for a workpiece:

 

• The characteristic blackening produced during burnishing makes the steel look antique, and is considered a visual enhancement.
• Burnishing protects against corrosion, especially from oils and greases
• The burnished layer is highly resistant to abrasion and bending and offers temperature resistance up to 300 °C
• Hardly any impairment of dimensional stability, and thus no warping
• No impairment of conductivity

Burnishing is used in mechanical and plant engineering to increase the service life of components. The process is therefore of major importance, especially for roller bearings, and also for springs, clamping jaws, screws, fittings and controls as well as for components used in handguns. In addition, parts are burnished for aesthetic reasons to blacken them, and thus to give them an antique look.

 

NFO: Blackening
Besides burnishing, there are other methods that fall under the category of blackening. Blackening is also used for the corrosion protection of forged products and iron cookware. For this purpose, linseed oil is burned off on the iron surface. The workpiece is heated to a temperature of 400 °C to 700 °C in glowing coal and then quenched with linseed oil, or else coated with oil first and then heated. The oil is dissipated in the form of smoke. The process often requires several passes to produce the desired outcome. The process is also referred to as black annealing or dark marking.

 

Oxidisable metals such as steel, copper, brass, and cast iron are suitable for burnishing. Non-ferrous metals and rust-resistant steel are not suitable for burnishing because they cannot form a layer of mixed oxide.

 

An overview of some suitable materials can be found at the bottom of this page.

 

While chromating and phosphating involve the incorporation of their respective eponymous elements into the surface layer, only oxygen is incorporated during burnishing. This leads to the formation of the oxide layer, composed of mill scale or of iron (II) or iron (III) oxide.

 

DIN 8580 does not classify burnishing as a coating as per main group 5, but as a change in material properties as per main group 6.3. This means that particles are introduced into the surface instead of being applied to the surface. For this reason, the dimensions of the component remain almost unchanged.

 

The individual burnishing parameters have a decisive influence on the lasting quality of this surface treatment. A great amount of experience is necessary to strike the important balance between temperature, time, chemicals and concentration that will perfectly suit your workpieces. State-of-the-art systems ensure that the sensitive parameters are strictly complied with. We at Härtha are therefore able to guarantee your desired product characteristics, even with a very narrow tolerance range.

 

Maximum workpiece size:
2,700 x 400 x 600 mm

 

Our interactive location overview tells you which of our locations in Germany and Europe offer burnishing.

• A higher colour depth can be achieved by multi-stage burnishing (DIN 50 938)
• Only iron oxidises during burnishing – which is why the iron content determines the depth of the blackening. As a consequence, a higher concentration of other alloy components leads to lighter and possibly reddish colourations
• When applied to stainless steels, burnishing is referred to as blackening or stainless steel blackening
• Due to the chemical nature of the process, internal areas and holes can be burnished without a problem
• During what is known as rapid burnishing, the burnishing solution is applied manually to the workpiece.

 

We are delighted that you wish to commission burnishing from us. Please start by providing us with the material designation and information about any thermal pre-treatments.