Improvements In Thermal Hard-Coating Technology
Thermal CVD hard coating, a process developed in the 1950s, is used to deposit hard ceramic coating materials on metallic substrates. Recently, the CVD hard-coating process has been optimized in a process called high-performance hard coating, which offers improved performance via a combination of several specific components and technical solutions. In the CVD process, different heterogeneous reactions and intermediate reactions occur between coating source materials and the steel surface to be coated. A thorough study of these complex chemical reactions and process mechanisms led to an optimization of process equipment and deposition parameters for HPHC. The result is a general change of almost all important deposition parameters, such as gas amounts and mixtures, concentrations and partial pressures of TiCl4 (the hard-coating metal source material) and other metal component sources together with their distribution in space and time.
The HPHC process is carried out at slightly reduced, uniform temperatures even in large volume reactors, yielding enhanced deposition rates. Much less distortion and size change of steel parts occur than in the case of classic high-temperature CVD processes. A major industrial CVD application is hard coating of cemented carbides, because no additional heat treatment is necessary. However, steel parts must be heat treated in most cases after thermal CVD coating, which can cause distortion due to thermal expansion mismatch between the steel and hard coating. The uniform layer thickness and uniform process temperature of the HPHC process contribute to maintaining the size and shape of coated precision parts during the required post-coating heat treatment. However, the heat treatment of coated parts is somewhat more complicated.
HPHC technology is a clean gaseous process, carried out in a closed reactor and gas neutralization system, which eliminates environmental problems and requires no waste management. The common TiN, TiCN and TiC hard-coating materials and their combinations can easily be deposited using HPHC technique with or without TiCl4 as the metal source.
Most currently applied hard-coating materials still are based on titanium, but other refractory materials and several combinations thereof also are available. HPHC can use other source refractory metals, as well as selected carbon supplying chemical compounds. Using specific HPHC equipment, several metal sources of interest can be transported into the reactor zone. This is possible for elements such as the IVB (Ti, Zr, Hf), VB (V, Nb, Ta) and VIB (Cr, Mo, W) groups in the periodic system of elements and a few others. At appropriate temperatures, these metals react with carbon, nitrogen and oxygen containing compounds, yielding their corresponding hard materials; i.e., carbides, nitrides and oxides, respectively. HPHC technology offers an easy, practical way to routinely deposit hard-layer systems having improved performance and variability. These materials have a wide range of properties, which will open up a new spectrum for improvements of functional coatings in industrial applications.
High-performance hard coating
Titanium-base standard hard layers (TiN, TiC and TiCN and various combinations) typically are deposited using TiCl4 as a metal source, which is easily handled and evaporated. The most common layer combination is TiC-TiCN-TiN because the deposition of TiC on steel at elevated temperatures is supported by the diffusion of carbon out of the steel. Therefore, it is relatively easy to start the coating process with a fast growing TiC layer and then change later on through an intermediate gradient TiCxNy (x = 1_0; y = 0_1) composite zone to a TiN cover layer.
TiN also can be deposited without any major complication, but high internal stresses can develop leading to lateral and longitudinal fractures in the layer[3, 4]. The TiN-TiCN-TiC layer system has a harmonic character and significantly better stability than TiC-TiCN-TiN (refer to tables and figures). The improved superior characteristics of the TiN-TiCN-TiC layer system on steel was predicted about 20 years ago , but the complicated deposition of TiC on TiN was not commercialized. Because TiN functions as a diffusion barrier for carbon, a TiC layer cannot easily be deposited on the top of TiCN or TiN under common conditions. The necessary amount of carbon must be supplied via the gas phase using a mixture of reactive gases. HPHC technology offers an easy, practical way to routinely deposit a TiN-TiCN-TiC hard-layer system having improved performance.
Differences in thermal expansion cause compressive and tensile stresses at the different layer interfaces and the coating/steel interface after cooling down from a coating deposition temperature of about 1000C (1830F). The lowest difference occurs between a TiC/TiN and TiN/steel interface. The largest difference (__ = 6 x 10-6/C) occurs between a TiC/steel interface, which poses the greatest risk of breaking, fracture and layer separation[3, 4]. The difference between TiN/steel and TiN/TiC interfaces is only one-half that of TiC/steel.
It is difficult to equalize or compensate for distortions created from coating a finished part. Because CVD and HPHC coating usually is carried out above the tempering temperatures of steel, an additional heat treatment of coated parts is necessary. However, there are some steels and heat treatments available that allow exact control of part size and shape.
Similar titanium hard-coating materials can have significant differences depending on how they are formed and the deposition technique used. For example, TiN deposited using HPHC and CVD techniques have markedly greater wear resistance and longer service life compared with TiN material deposited via PVD technique. However, PVD and CVD techniques complement rather than compete with each other in different applications. CVD and PVD hard-coating advantages and applications are given in. PVD layer thickness is limited to about 5 Km for good adherence, while CVD layer thickness to over 10 Km presents no problems.
A highly uniform coating is possible within the entire reactor using HPHC deposition parameters. Layer thickness uniformity typically is better than I5%, and is better than I10% over a total length of 1.5 m (5 ft). Coating uniformity in small bores; deep, dead-end holes; tight slots and relief areas also is significantly improved.
The hardness of TiC applied using CVD typically is between 3200 and 3600 HV. Hardness can be increased to between 4000 and 4300 HV using the HPHC technique plus adding specific impurities. TiC cannot be deposited using PVD.
Reliable performance of coated parts and tools depends on several major factors in CVD and HPHC coating technologies including the use of relatively high deposition temperatures and the final stress set up in the coating. Therefore, the combination of thermal expansion values and layer thicknesses plays an important role (refer to tables and figures).
Many different factors interact when friction occurs under high surface pressure including coating adhesion, elasticity, brittleness and friction coefficient of the layer material, as well as the type of lubricant, if used. Friction under high pressure can lead to local high spot temperatures at peaks on an uneven surface, where metal can partially melt and weld to the mating surface. This situation results in galling and surface degradation. Due to the ceramic nature of carbide and nitride hard coatings, no galling occurs because molten metal does not weld to the ceramic surface.
Wear of the softer material potentially will occur for mating materials having different hardnesses. The soft side will always be replaced, while the hard side (functioning as a tool) will achieve improved lifetime. Wear resistance and life of hard layers depend on their own hardness, friction coefficient and other interactions, and also on their thickness, known as "wear reserve" or "wear capacity."
Other aspects that must be taken into consideration in addition to those mentioned above with respect to service life are materials interactions including tribochemistry, properties of the carrying substrate, power, pressure, acting forces and their directions, etc. Generally, wear resistance increases significantly in ascending order from nitriding, boriding, PVD- and CVD-coatings up to HPHC treatment.
CVD and HPHC hard coatings significantly increase part performance and service life in wear, erosion and abrasion applications because the ceramic-like properties of hard-coating materials on metal surfaces minimize direct metallic interaction with steel or other metals. The coatings also improve the efficiency of some special tools and machine components, and reduce lubrication requirements. Typical applications include tooling in the metals industry (cold and warm metal forming, deep drawing, cutting, aluminum extrusion, die casting) and plastics industry (extrusion of polymer matrix composites; cylinder bushings and extruder screws of injection moulding machines, nozzles, etc.); heavy-duty machine parts; components subjected to heavy mechanical loading and harsh chemicals; and cutting tools and blades used in food and wood processing. IH
SIDEBAR: Thermal CVD: The beginnings
Thermal CVD hard coating of titanium-base materials on other material substrates was developed in Germany in the 1950s. Titanium tetrachloride (TiCl4) was reduced using hydrogen at an elevated temperature in a reactor containing nitrogen gas according to the reaction: TiCl4 + 2 H2 + 1/2N2 = TiN + 4 HCl. The reaction produced a yellow hard material (identified as titanium nitride, or TiN), which was deposited on the inner surface of the vessel.
Similar reactive conditions in a nitrogen-free chamber resulted in the reaction: TiCl4 + CH4 + (H2) = TiC + 4 HCl + (H2).This produced a silver-grey hard material (identified as titanium carbide, or TiC), which was deposited on carbon steel. TiC also can be produced by adding a carbonaceous gas, such as methane (CH4) or other hydrocarbons[8, 9].
The first-generation CVD hard-coating reactor was a pit-type reactor vessel used for experiments at the Metallgesellschaft. The entire reactor had to be moved together with its workload into and out of a pit furnace for each treatment, and, therefore, required flexible connections. Pit-type CVD plants typically operate at atmospheric pressure with high throughput and low efficiency of the gases. Coating layer thickness is strongly limited by the consumption of TiCl4 as source material of titanium, resulting in decreased uniformity and layer thickness in the gas flow direction. This limited reactor size.
The second-generation CVD plant was a fixed place bell-type reactor having stationary firm connections, which made it easier to use reduced inside pressure. This method provides better uniformity of layer thickness distribution, but a slightly reduced deposition rate.
Both reactor types were developed empirically, based on the estimated process requirements of supposed importance[12-14]. Both concepts apply thermal gradient heating to compensate for nonuniform deposition rates.
In the early 1950s, TiN and TiC hard-coating materials competed with each other, but also complimented each other due to their different properties[9, 15]. Other hard materials (mainly carbides of chromium and tungsten) having specific properties also were generated and deposited. Chromium carbide (CrC) generated great interest because its thermal expansion coefficient was nearly equal to that of steel. In addition, chromium-base hard layers enhanced corrosion resistance[15, 16]. However, the necessary conditions for chromium deposition were difficult to control.
For more information: Andreas Szabo is chemist, consultant, Best-Surface Consulting, Simanowizstr. 12, D-71640 Ludwigsburg, Germany; tel: (+49) 7141 242 0810; fax: (+49) 7141 242 0811; e-mail: firstname.lastname@example.org; or contact David Pye, Pye Metallurgical Consulting, Meadville, Pa.; tel: 814-336-1028; fax: 814-337-5939; e-mail: email@example.com
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