ICMCTF2000 Session E2-2: Wear Resistance of Ceramic, Metallic and Composite Coatings

The takeup of thin hard wear resistant coatings in engineering applications has been impeded by the lack of robust methods for assessing their wear and friction performance in the laboratory. Considerable progress has been made in the last few years to this goal with the development of the ball cratering or microabrasion test.

A ball is rotated and pressed against the coated test sample. An abrasive is introduced between the ball and the test sample, and after a fixed number of revolutions the size of the crater that is formed can be used to calculate the wear that occurs. If a series of craters is made for a range of test durations, then wear rates for both the substrate and the coating can be derived.

This test is proving to be suitable for testing the resistance of thin hard coatings to wear by fine abrasives. The test has advantages that it is quick, convenient, it can be performed on small samples, and uses relatively inexpensive equipment. There is still, however, a need for tests which can be used to evaluate the sliding wear and friction performance of coatings. Although conventional tests such as pin-on-disc tests have been used many times in the investigation of friction and wear of thin hard coatings, there are a number of problems with their use and interpretation.

This paper describes a simple modification of the normal ball cratering test equipment which enables these experiments to be carried out with a rotating ball against flat geometry. Tests can be carried out both dry or with lubricants. Friction is measured by means of strain gauged flexure elements. Friction and wear are recorded throughout the test.

The utility of the test system is illustrated by the results of a number of preliminary experiments on a range of thin hard coatings including TiN, MoS₂, DLC, and ZrN. The test results are augmented by opticcal and SEM observations of the wear mechanisms that occur.

The formation of transfer layers during turning and milling was studied. Initially 42CrMo4 was used as workpiece material in all cases. Later on a Ca treated variant of the 42CrMo4 steel was also used for the turning tests . All tests were done under dry as well as under lubricated conditions. Two lubricants were selected for the tests: a conventional lubricant and an ecological lubricant. The turning tests were done with uncoated, TiN- and (Ti,Al)N-coated cermets while for the milling tests uncoated, TiN-coated and Ti(C,N)-coated inserts were selected.

An analysis of the tool surfaces was made by XPS measurements while FTIR and FT Raman spectroscopy were done to study the tool surfaces used under lubricated conditions. Since end of tool life mostly coincides with severe damage to the cutting edge it was decided to stop all tests before end of tool life was reached. In doing so it was expected that the detection of transfer layers would be facilitated.

In order to protect machining parts against wear and corrosion, they are coated by cermet coatings. The coatings consist of WC or Cr₃C₂ particles in a metal binder, which can be a pure or a mixture binder consisting of Ni, Cr, Co. Two thermal spraying methods were used to produce cermet coatings: high velocity oxy-fuel and high velocity air flame spraying. The coatings were investigated with respect to erosion and corrosion resistance. The corrosion/erosion tests were carried out in synthetic sea water containing sand. The corrosion rates were determined by electrochemical measurements. The results show that the corrosion properties of the coatings strongly affect the materials loss rate in both tests. Coatings with a less corrosion resistant matrix present enhanced erosion.

The samples were examined by light microscopy (LM), scanning electron microscopy (SEM) and X-ray diffraction (XRD).

With the introduction of stricter environmental laws governing the usage and disposal of used metal cutting fluids the current trend is towards dry machining. In order to meet the severe demands of dry machining research has focussed on advanced surface coatings that have increased resistance to wear and high temperature oxidation. However, critical to the economic viability of dry machining is justifying the increased tool costs in terms of productivity and ecological savings achievable through the elimination of cutting fluids.

The introduction of PVD coatings to cutting tools has enabled improved cutting tool performance at increased cutting speeds and feed rates. However, it has been shown that at temperatures exceeding 550°C titanium nitride (TiN) coatings oxidise to the brittle compound TiO₂ and tool life is adversely affected. At the forefront in the development of high temperature PVD coatings has been titanium aluminium nitride (TiAlN) which has shown superior performance over traditional vapour deposited coatings such as TiN when used in dry machining. The additions of aluminium to TiN coatings have been shown by Tönshoff et al. to strengthen the cubic face-centered structure of the coating by partial substitution of titanium atoms with smaller aluminium atoms. This strengthening in the TiAlN fcc lattice results in improved high temperature strength, higher oxidation resistance and better thermal barrier properties compared with TiN.

This paper presents the results of dry drilling P20 low alloy steel and automotive gray cast iron using partial and fully filtered TiAlN coated Co-HSS and carbide drills. Emphasis is placed on evaluating the optimum cutting speed and feed rates for dry drilling with TiAlN coated drills as well as studying the effect of coating surface roughness and macroparticle content on drill life.

Compact, high-pressure reciprocating compressors developed primarily for NGV-applications (NGV = Natural Gas for Vehicles) are capable of compressing several other technical gases from atmospheric pressure to 200 bar under oil-free operating conditions. The heart of this device is a four-stage, high-pressure compressor head comprising unlubricated freely floating pistons in the cylinder with a tight tolerance on gap width. Gases, such as helium, argon, dry nitrogen and hydrogen require special material combinations and/or coatings for the piston/cylinder assembly. Novel graded and multilayer coatings, including MoS_x/Ti, diamond-like carbon (DLC) and derivatives were tested in several combinations under service conditions.

The coatings were initially characterised with respect to thickness, hardness, surface roughness and chemical composition. After compressor tests more detailed investigation was performed using optical microscopy, x-ray photon spectroscopy (XPS) and scanning auger microscopy (SAM) in order to understand the failure mechanism and the influence of the service conditions on the coatings.

Two DLC coatings, produced at different process temperatures using the CVD technique, as well as a MoS_x/Ti coating were tested in dry natural gas and nitrogen.

After 4000 h operation in dry natural gas or in nitrogen no wear was detected on the DLC coatings, irrespective of deposition temperatures. Both the low temperature and high temperature deposited DLC coatings exhibited no evidence for nitrogen incorporation in the coating. A reduction of sp³ hybrid bonding detected by XPS in the low temperature deposited DLC after compressor testing was attributed to a graphitisation probably promoted by sliding contact during the test. However, although this may be expected from a metastable coating which is formed at low deposition temperatures, the graphitisation apparently did not influence the compressor performance. On the other hand, no decarburation was observed in the high temperature deposited DLC coating even after 4000h in nitrogen atmosphere.

MoS_x/Ti coatings showed a different wear behaviour which was strongly dependent on the counterpart material. MoS_x/Ti coatings sliding against uncoated steel tended to scuff after a short running time. In the case of MoS_x/Ti coatings sliding against a DLC counterpart, after 4000 h of running time break-out of micro-platelets and a concomitant loss of compressor power were observed. The titanium oxide, which is believed to contribute to the coating hardness, was found to be present in the form of MoTiO₃ and TiO_x compounds.

A reactive hybrid physical vapour deposition process, utilising a combination of electron beam evaporation and DC magnetron sputtering, was used to deposit various single and multilayered coatings on high speed steel substrates. Homogenous TiN, TaN and TiAlN coatings, as well as TiN/TaN, TiN/NbN and TiN/CrN multilayered coatings were grown. The deposition procedures are described in more detail in the paper.

Particle erosion studies using silicon carbide particles as an erodant have been performed. For test series one, impingement angle, particle size and particle velocity of 40°, 20 µmm and 25 m/s, respectively, was chosen in order to avoid any deformation of the substrate. A second test series, identical to the first with the sole exception of the particle size being 200 µmm instead of 20 µmm, was also carried out. On this basis, erosive wear resistance of the different coating materials has been determined. Furthermore, to better understand the mechanisms associated with the wear of these films and to evaluate the importance of layer toughness in multilayered coatings on erosion resistance, all worn films have been investigated using scanning electron microscopy.

The results show that TiN/NbN, TiAlN and TiN/TaN displays the best particle erosion resistance of the investigated coating materials, independent of particle size.

Keywords: Coating; Erosion; Multilayer; PVD; Tribology; Wear