D.B. Lewis

Recent progress in large scale manufacturing of multilayer/superlattice hard coatings

Since the early fundamental research on superlattice structured hard coatings in the late 1980s, rapid progress has been achieved to produce nanoscale compositionally modulated multilayer structures. It has been shown that the periodicity of the multilayers is strongly controlled by the substrate rotation and the actual deposition rate. Appropriate multi-target geometry and controlled target poisoning by optimised pumping conditions lead to deposition conditions similar in their economy to the deposition of typical monolithically grown binary hard coatings. The combined steered cathodic are/unbalanced magnetron technology guaranties sufficient adhesion (L-C > 50 N) of the usually highly stressed coatings as well as smooth surfaces due to UBM deposition (R-a m). This paper has described the properties of coatings dedicated to high temperature performance: TiAlN/CrN (period 3.03 nm), to tribological applications: TiAlYN/VN (period 3.62 nm) and combined wear and corrosion resistance CrN/NbN (period 3.2 nm). All the coatings investigated were found to crystallise into B1 NaCl f.c.c. structures, and exhibited {110} and {111} or {100} preferred orientations for TiAl/CrN, TiAlYN/VN and CrN/NbN superlattice coatings, respectively. The residual stress was found to be compressive in the range of -4.0 to -8.5 GPa for TiAlYN/VN and between -1.8 and -7.5 GPa for CrN/NbN, depending on the stoichiometry and the bias voltage during coating deposition. Corresponding to the high stress values, the plastic hardness of the coatings was measured to be 55-60 GPa for TiAlN/CrN, 42-78 GPa for the TiAlYN/VN system and between 42 and 56 GPa for CrN/NbN, depending on the bias voltage. Oxidation resistance at temperatures exceeding 900 degreesC was typical for TiAlN/CrN. The TiAlYN/VN coating showed superior tribological properties with a coefficient of friction mu = 0.4 and low sliding wear of 1.26 X 10(-17) m(2) N-1 after 1.1 million cycles against an Al2O3 ball in a pin-on-disc test. CrN/NbN exhibited two orders of magnitude lower passive current densities than electroplated hard Cr and a pitting potential of 450 mV during polarisation in acetate buffer solution. When Nb+ ion etching was used, the CrN/NbN superlattice coating deposited on 304L, stainless steel showed high pitting potentials in the range of 750-1000 mV in the same corrosive medium

Low temperature plasma diffusion treatment of stainless steels for improved wear resistance

Abstract The use of a thermionically enhanced low pressure plasma process to provide nitriding and carburizing treatments on a precipitation hardening martensitic stainless steel (AISI 17/4PH) at low temperature (420°C or less) is reported. The resulting diffusion layers are analysed by glow discharge optical spectroscopy and X-ray diffraction depth profiling to provide information on the changes in diffused-species concentration and metallurgical structure with treatment depth. It is shown that there exists the possibility to synthesize a variety of layered structures under different plasma conditions using the low pressure enhanced plasma process—with particular emphasis on the production of hard, yet “precipitate-free” surface layers based on an expanded austenite lattice. These types of layered diffusion treatments may provide considerable improvements in the wear resistance of stainless steels without significantly compromising their desirable corrosion-resistant properties.

Chromium nitride coatings grown by unbalanced magnetron (UBM) and combined arc/unbalanced magnetron (ABS™) deposition techniques

Abstract Increasingly chromium nitride coatings are being used as a replacement for electroplated hard chromium in various applications. This paper reports an investigation of chromium nitride coatings deposited at 250°C on various substrates at different N2 partial pressures using both UBM sputtering and ABS techniques. The effect of process parameters on structure, composition, hardness and adhesion have been investigated by XRD, SNMS, SEM and a range of mechanical testing techniques. The presence of Cr, Cr + N, Cr2N, CrN, and mixtures of the phases have been identified and related to both the film composition and the process parameters. Substoichiometric Cr2N films had the greatest hardness with values up to 2100 HV. However, films of this hardness exhibited poor adhesion with critical loads, Lc, of 30 N (HSS) and HRc-DB of “4”, if deposited with UBM only. X-ray diffraction indicated that the poor adhesion exhibited by these films was associated with high internal stress. The adhesion of substoichiometric films with comparable composition and crystal structure deposited in combination with metal ion etching (ABS) increased the critical load values up to 60 N (HSS) and improved the HRc-DB to “1” (HSS).

Droplet formation on steel substrates during cathodic steered arc metal ion etching

Abstract Cathodic arc discharges are commonly used as metal ion sources to carry out, in vacuo , the precleaning procedure in cathodic arc evaporation and combined cathodic arc/unbalanced magnetron deposition processes. During this very efficient etching step, droplets are generated, which give rise to growth defects in the subsequently deposited hard coating. The number and size of droplets depend on the melting point of the metals used during ion etching. The present paper investigates the generation of droplets by target materials Al, Cu, TiAl, Ti, Zr, Cr, Nb and Mo with melting points in the range 660–2650 °C. Under the process parameters used, the TiAl alloy target showed the highest number of droplets generated (100 × 10 3 mm −2 ) whereas the largest, up to 20 μm diameter and quite often ‘splash like’, formed during evaporation from Al targets. Both metals with melting points greater than 2000 °C show very similar droplet generation, with mean droplet deposition densities of 5−10 × 10 3 mm −2 and diameters up to 5 μm. A comparison between the as-etched and as-coated samples indicates that the number of droplets deposited during the etching phase with TiAl and Mo formed an identical number of growth defects. In the case of Cr the number of defects on top of the coated surface were less. In general, following a non droplet producing unbalanced magnetron deposition process, defects were observed to extend from the substrate/coating interface to the coating surface.

Structural determination of wear debris generated from sliding wear tests on ceramic coatings using Raman microscopy

In order to address the important interest in wear debris and associated wear mechanisms, we have studied a series of physical-vapor deposition ceramic hard coatings (CrN/NbN, CrN, NbN, TiAlN/VN, and TiCN) using a ball-on-disk sliding configuration against corundum. The debris generated were characterized using Raman microscopy to identify compounds, especially oxides, generated during the wear process to gain a better understanding of tribochemical reactions. The high spatial resolution (2 μm), sensitivity to structural changes, and nondestructive nature make this technique ideal for the study of such small amounts of wear debris. This article examines binary, multicomponent, multilayered, and superlattice coatings. Under dry sliding conditions of 5 N normal load and 10 cm/s speed, titanium-based alloy coatings were found to provide TiO2 (rutile) debris. However, the addition of thin layers of VN to the TiAlN system provided a lower friction coefficient, and much less debris through the possible formatio...

Chromium nitride/niobium nitride superlattice coatings deposited by combined cathodic-arc/unbalanced magnetron technique

CrN/NbN superlattice coatings have been developed as an attempt to replace electroplated chromium in some applications. The coatings have been deposited by a combined cathodic-arc/unbalanced magnetron technique in an industrial-size physical vapour deposition (PVD) coater. The investigations have been focused on the question of maximum hardness, adhesion and tribological performance of the coatings deposited at 400 degrees C as a function of the nitrogen content in the films. All CrN/NbN superlattice coatings produced exhibit a single-phase face-centred cubic structure and {200} preferred orientation. The superlattice period of the coatings varies in the range Delta = 3.4-7.4 nm depending on the N-2 flow rate. Under the sputtering conditions used, it was possible for stoichiometric CrN/NbN coatings to be deposited only in a narrow N-2 flow rate range of around 160 seem. Both stoichiometric and sub-stoichiometric coatings showed maximum hardness values of Hk = 3580 and Hk = 3600, respectively. CrN/NbN superlattice coatings outperformed electroplated chromium by factor of 13 in dry sliding conditions. However, both coatings show similar abrasive wear in the range of 0.63 mu m N-1. Stoichiometric CrN/NbN superlattice coatings possess high oxidation resistance in the range 820-850 degrees C

Large-scale fabrication of hard superlattice thin films by combined steered arc evaporation and unbalanced magnetron sputtering

Abstract This paper reports on the mechanical and physical properties of a range of TiAIN- and TiN-based polycrystalline superlattice hard coatings fabricated by a combined steered arc evaporation and unbalanced magnetron sputter industrial batch process within a common gas atmosphere. The reactive deposition experiments were carried out in a four-cathode system in which the substrates could be continuously rotated at a nominal target-substrate distance of 250 mm and with variable velocity. The influence of layer composition, deposition technique, substrate rotation and coating rate were investigated by XTEM, XRD, RBS and Knoop indentation testing with regard to the superlattice period and microstructure, and the preferred orientation and hardness of the coatings. All coatings were found to exhibit fine, highly dense lamella microstructure, high hardness and excellent adhesion. Film systems deposited by closed-field unbalanced magnetron sputtering could exhibit preferred 〈111〉 growth, whilst coatings deposited by simultaneous steered arc/unbalanced magnetron sputtering generally showed 〈200〉 orientation. The period of the superlattice could be controlled between 19 and 172 A by alteration of the substrate rotation and deposition rate.

Structure and stress of TiAlN/CrN superlattice coatings as a function of CrN layer thickness

Abstract TiAlN/CrN superlattice coatings have been grown in an industrial sized multiple target PVD coater by a combined steered arc/unbalanced magnetron process. The coatings were deposited using three Ti 0.5 Al 0.5 alloy targets and one Cr target. The power to each of the Ti 0.5 Al 0.5 targets was maintained at 8 kW in all the processes, whilst the target power to the Cr target was varied between 1 kW and 12 kW to produce different superlattice period thickness. The structural characteristics were determined by X-ray diffraction using both Bragg–Brentano and glancing angle parallel beam (sin 2 ψ method) geometries. Superlattice period thickness was determined using low angle X-ray diffraction and was found to increase from 2.2 nm at 1 kW Cr power to 4.8 nm at 12 kW Cr target power. An approximately linear dependence of Cr power on superlattice period thickness was also observed. A maximum Knoop hardness of HK 0.025 3500 was observed at 8 kW Cr power when the thickness of the individual layers was equal. The residual state of stress was found to be compressive in all cases rising from 3.25 GPa at 2 kW Cr power to 6.5 GPa at 10 kW Cr power. Lattice parameters were determined using the Cohen–Wagner method and were observed to increase from 0.4183 nm for coating deposited at 2 kW Cr power to a maximum of 0.4197 nm at 10 kW Cr power.

Investigation into the structure of electrodeposited nickel-phosphorus alloy deposits

Abstract This paper presents a study into the structure of NiP deposits containing 1.15–15.43 mass% P using X-ray diffraction. The coatings were deposited from electrolytes containing both phosphorous and phosphoric acids using conventional direct current. Samples obtained by electroless deposition were also examined for comparative purposes. The effects of composition on crystallite/grain size have been determined quantitatively using an X-ray diffraction line broadening technique and the formation of X-ray amorphous structures have also been followed using X-ray diffraction. The development of the amorphous phase, however, was found to be sensitive to crystallographic orientation. At amorphous contents above 70%, the crystalline nickel phase was associated only with (111) (222) orientations parallel to the surface of the deposits. This appeared to result from a decrease in the growth rate of other orientations, e.g. (200), relative to the (111) orientation.

Metallurgical study of low-temperature plasma carbon diffusion treatments for stainless steels

Abstract We recently reported a novel low-temperature carbon diffusion technique forsurface hardening of stainless steels. The treatment was shown to provide benefits in terms of abrasive wear resistance. There is also evidence to suggest that by performing diffusion treatments at low temperatures ( i.e. below 400°C), these benefits can be achieved without compromising corrosion resistance. Here a variety of surface analysis and depth profiling techniques have been used to determine the physical and mechanical properties of carbon-rich layers produced on a range of stainless steel substrate materials. X-ray diffraction (XRD) was employed to determine the crystallographic structure, whilst wavelength dispersive X-ray analysis (WDX) and glow discharge optical spectroscopy (GDOS) gave information on the concentration and distribution of the diffused species within the treated layers. A variety of carbide-based structures was detected, including the expected M 23 C 6 and, more surprisingly, M 3 C. Optical and electron microscopy techniques were used to provide information on layer morphology. The surfaces produced by the low-temperature carbon-diffusion process generally exhibit a distinct diffusion layer of between 1 and 20 μm, depending on the material and the treatment conditions. Austenitic stainless steels appear to give the best response to treatment, however other types of stainless steel can be treated, particularly if the microstructure contains above 5% retained austenite. Here we discuss the changes in mechanical and metallurgical properties provided by this technique and its potential value for treatment of both austenitic and other stainless steel substrate materials.