Tantalum nitride (TaxN1-x) coatings were deposited onto high-speed steel using reactive DC magnetron sputtering. Five different combinations of magnetron power and nitrogen flow were used. Phase analysis (XRD) revealed that the phase composition changes with increasing N-2 flow from a mixture of Ta and Ta2N to single phase Ta2N. This result was supported by GDOES analysis of the chemical composition. The coating hardness and residual stress were both found to increase with the amount of Ta2N in the coatings. In scratch testing, it was not possible to provoke adhesive failures at loads up to 100 N. Finally, the coating abrasion resistance was very high for all specimens and found to increase with the amount of Ta,N in the coatings. (C) 1997 Elsevier Science S.A.

The tribological performance of thin hard coatings is, for a given substrate material, mainly governed by the coating hardness, coating fracture resistance, the contact temperature and chemistry (in the prevailing tribosystem). For a given application, an improved tribological performance can therefore, for example, be accomplished by increasing the coating fracture resistance while retaining the hardness, or vice versa. Four different PVD TiN/NbN multilayer coatings were deposited on high speed steel and cemented carbide substrates. Homogeneous TiN and NbN coatings were used as references. The coated composites were investigated with respect to coating thickness, morphology and microstructure, adhesion, hardness, residual stress and resistance to abrasive and erosive wear. The investigation showed that the deposition process works well for the deposition of well adhering, multilayered TiN/NbN coatings. The highest fracture resistance was found for the multilayer coating deposited with the thinnest (5–10 nm) individual layers of TiN and NbN. This implies, in combination with the high hardness of the 10/5 coating, that a multilayered coating has the potential to be both tougher as well as harder than a single-layered coating provided that the multilayer consists of very thin (≈5–10 nm) layers.

In this work, a fast and easily performed four-point bending test for evaluation of cracking resistance of thin hard coatings is presented. A bending device, small enough to be put in an SEM and observed in situ, has been designed. By crack formation studies, a measure of coating cracking resistance is obtained. Two methods of crack detection are utilised: detection of acoustic emission and direct observation in the SEM. The two methods yield virtually identical values of the cracking resistance. However, the acoustic evaluation is much faster and easier to perform and is therefore to be preferred in long test series. SEM observations, on the other hand, allow a more straightforward interpretation. In this paper the test is used to determine the coating strain corresponding to crack initiation in TiN and CrN coatings on high speed steel. The test yields values of cracking resistance for the coatings in the range 0.1% (TiN) to 0.7% (CrN). In the SEM studies, cracks were found to nucleate predominantly at defect sites in the coating and propagate, highly aligned, perpendicular to the length of the beam. The cracks usually terminated when reaching the substrate. However, if hitting carbide at the substrate-coating interface, the crack also continues through the carbide, and terminates when reaching the substrate matrix.

The wear characteristics of PVD Ti/TiN multilayer coatings subjected to two-body abrasion and particle erosion have been studied using diamond slurry and silicon carbide particles as abrasive medium and erodant, respectively. The abrasive wear rate of the Ti/TiN multilayer coatings was found to increase with the relative amount of metallic Ti in the coatings. In erosion, the lowest wear rate was recorded for the homogeneous TiN coating. For the Ti/TiN multilayer coatings the erosion rate was found to decrease with an increasing relative amount of metallic Ti in the coatings. It is concluded that the concept of multilayered coatings offers a potent means to tailor the properties of tribological coatings. In particular, demands of different applications can be met by adjusting the relative thickness of metallic Ti in Ti/TiN coatings. The amount of metallic Ti can, for example, be used to control the coating residual stress state. Multilayered Ti/TiN coatings seem promising for combined wear and corrosion protection.

In the present study the influence of substrate material on the erosion resistance of three different TiN coated tool steels (two high speed steels and one cold work steel) has been investigated. The particle velocity was 20 m s(-1), two angles of impingement (20 degrees and 30 degrees) were used and silicon carbide was used as erodent.The results showed that, the carbide volume fraction and the impact toughness of the substrate material controlled the erosion rate of both coatings and substrates. It was concluded that as long as the test parameters allow the impinging particles to significantly affect the substrate material during erosive testing of a coated composite, the substrate material will influence the erosive response of the coating.

PVD TiN and CrN coatings were deposited at both low (approximate to 200 degrees C) and standard temperatures (approximate to 400 degrees C) using reactive arc-evaporation Variations in microstructure (obtained by TEM and XRD), morphology (studied using SEM) and chemical composition (determined by both EDS and GD-OES) were correlated to mechanical (residual stress state, microhardness) and tribological properties (scratch response, abrasive wear resistance).The results show a significant increase in hardness and residual compressive stress for the low-temperature TiN coating as compared with the standard-temperature coating. This was attributed to a drastic decrease in grain size and an increased compressive microstrain for the low-temperature coating. Moreover, the critical load and abrasive wear resistance of the low-temperature TIN coating had decreased as compared with the standard-temperature coating, although the hardness was significantly higher. This was due to the poor quality of the low-temperature TiN coating in combination with the high residual compressive stress.It is further shown that mechanical and tribological properties of the low-temperature CrN coating were comparable with the standard-temperature CrN coating, even though there was a difference in microstructure and chemistry.

In the manufacturing industry, there is a pronounced economic interest in the recoating of used tools. The influence of recoating on the mechanical (composite hardness, scratch test response and residual stress) and tribological (intrinsic abrasive wear resistance) properties of TiN-coated high speed steel has been studied for two different deposition techniques. The overall mechanical and tribological performance of recoated specimens was found to be as good as single-coated TiN. For arc-evaporated coatings, the residual stress was found to decrease with increasing number of recoatings, whereas no influence was found for ion-plated coatings. In both cases, the coatings abrasive wear resistance was found to increase with decreasing coating residual stress.

This work aims at the development of multilayered coatings with increased fracture resistance, retained hardness and adhesion (compared with current state-of-the-art coatings) to the substrate. One way of obtaining this effect is to deposit multilayered coatings consisting of alternating thin layers of hard and softer, more ductile materials. A modified commercial PVD deposition process was used for deposition of multilayered Ti−TiN coatings on both high speed steel (HSS) and cemented carbide (CC) substrates in order to explore this. The multilayer coatings were evaluated with respect to fundamental properties such as morphology, microstructure, hardness, adhesion and fracture resistance. The deposition process was found to yield well-adhered coatings with an increased fracture resistance due to the multilayered structure, on both HSS and CC substrates.

Four different PVD coatings; CrN, TiN, (Ti, Al)N, and Ti(C,N) have been evaluated with respect to their mechanical and tribological properties. Two different tool steels were used as substrate materials. Several modern analytical methods, including glow discharge optical emission spectroscopy, and three different aboratory wear tests (erosion, abrasion, and sliding wear) have been used. The investigation shows that in order to minimise wear in erosion of coated parts, a coating–substrate composite with high substrate and coating hardness and sufficiently high coating thickness should be chosen. In particular, it was found that theinfluence of coating hardness increases with substrate hardness. A high coating fracture toughness also aids erosion resistance. The difference in intrinsic abrasive wear resistance between the investigated coatings is small. This implies that other coating properties, e.g. oxidation resistance etc., are the limiting factors whenone of these coatings is selected for a given tribological application. From the results of the sliding wear studies it was concluded that the amount of wear of the coated specimen decreases with increasing coating fracture toughness and coating hardness. To retain these properties at high sliding speed the chemical and thermal properties of the coating-substrate composite are highly important. The present work highlights the importance of the coating fracture toughness. This is a property that needs to be more closely examined and, in particular, there is a clearly defined need fordevelopment of reliable, easy to use experimental methods for fracture toughness determination. It should also be noted that the overall performance of the (Ti,Al)N coatings proved to be the best of all tested coatings, which means that if an uncertainty as to which coating material to choose arises, (Ti,Al)N is likely to be a good first choice.

Three potential processes for TiN deposition at low temperatures (high current density plasma beam evaporation (200-320-degrees-C), high current density plasma beam activated electron beam evaporation (220-degrees-C) and high current density plasma beam assisted sputtering (220-degrees-C)) are evaluated with respect to the resulting surface topography, hardness, scratch resistance and abrasive wear resistance of the deposited coatings. TiN coatings deposited using three different processes operating at standard temperature (450-degrees-C) were used as references.It is shown that it is possible to obtain low-temperature TiN coatings with retained properties (hardness, scratch and abrasion resistance) as compared with TiN coatings produced at standard temperatures. The results also indicate that, at low temperatures, activated evaporation seems to yield coatings with better mechanical and tribological properties than sputtering. An important observation is that the performance ranking of the investigated coatings differs with the parameters measured. This, in turn, implies that several methods must be utilized for evaluation of a candidate coating-substrate composite for a given application.

The influence of pre-coating substrate surface topography on the critical normal force obtained during scratch testing of four different TiN-coated tool steels has been studied. Six different surface preparation techniques (three based on grinding, and three on grit blasting) were used. The resulting surface topographies were characterized using scanning electron microscopy and surface profilometry.The critical normal force decreases somewhat with increasing surface roughness for two TiN-coated high speed steels, while no effect of surface roughness is found for coated hot- and cold-work steels. In the latter cases, an eventual influence of surface roughness is screened by the relatively large experimental scatter.The results of the present investigation emphasize the importance of a thorough post-test scratch inspection and that the definition of "critical normal force" is important for the relevance of the scratch adhesion test. An important conclusion is that the critical normal forces frequently quoted as measures of the adhesion between well-adhering coatings and their substrates instead are measures of the ability of the coatings to resist deformation.