A review of the present state of art in hard coatings grown from the vapor phase

Abstract

Hard-coating materials range from ultrahard materials such as ‘‘diamondlike carbon’’ through refractory compounds to alloys. However, transition-metal carbides and nitrides have achieved by far the highest level of commercial success. Titanium nitride and titanium carbide are the most studied and used. In this paper a review of the hard coating literature is given and includes in addition to nitrides and carbides also oxides, borides, mixed compounds, metals and alloys, and ‘‘diamondlike’’ carbon coatings. Only coatings grown from the vapor phase are discussed. Some considerations involved in selecting coating/substrate combinations as well as basic concepts of hardness and hardness measurements are also given. For example, it is shown that in order to measure the hardness of the coatings correctly the ratio between the film thickness and the depth of the indentation has to exceed a critical value, which depends on the coating/substrate combination. For TiN on steel, the coating thickness has to be a factor of 5 larger than the indentation depth. For all types of hard coatings there is still a lack of knowledge on how nucleation and growth processes are affected by processing parameters and how the resulting film microstructure correlates to physical properties. Based on results presented in the literature, the existing knowledge about relationships between the microstructure and physical properties of hard coatings is discussed. Particular emphasis is placed on the role of microstructural features such as grain boundaries, nonequilibrium structures, impurities, and texture in controlling the film hardness. For example, voids and weak grain boundaries give rise to low film hardnesses whereas dense films with a high defect concentration can have hardnesses far above bulk values. Because the coatings in many cases are grown at high rates, low temperatures, and under the influence of an impinging ion flux, supersaturated solid solutions and entrapment of noble-gas impurities are common features. Such coatings exhibit high stress levels (most commonly of a compressive nature) and high hardness values.

Influence of deposition parameters on mechanical properties of sputter-deposited Cr2O3 thin films

“…For low penetration depths (i.e., less than 10% of the coating thickness), very low hardness and elastic modulus values of 2.6 and 111 GPa, respectively, were measured and coincide with a low density value (∼2.8 g/cm 3 ) and an increased strain rate prefactor b. This low density value is attributed to high porosity and voids, which result in a lack of coherence of the grains and, consequently, in a low hardness value 4 and a strong room temperature creeping, At higher penetration depths, the hardness, the elastic modulus and the strain rate prefactor tend toward the values of the Si substrate indicating the strong influence of the substrate on these values. Much higher density values (4.8-5.0 g/cm 3 ) are obtained for films with lower oxygen concentrations.…”

“…An explanation for this phenomenon is that the interdiffusion at the film/substrate interface is increased at higher deposition temperatures and strengthens the adhesion to the substrate. 4 The differences in the absolute values of the hardness and the elastic modulus between the results from the XP and the NHT apparatus (Fig. 4) origin from the differences in the analysis of the raw data.…”

“…The major drawback compared to the generally softer transition metal nitrides, which are nowadays standard in the industry of wear protective coatings, is the usually lower toughness and transverse rupture strength of the oxide coatings. 4 Nevertheless, Cr 2 O 3 coatings have already found several applications e.g. as protective coatings on read-write heads in digital magnetic recording units 5 or in gas-bearing applications.…”

Ab initio and classical molecular dynamics simulations of N2 desorption from TiN(001) surfaces

“…Due to remarkable physical properties including high hardness and mechanical strength [1][2][3], chemical inertness [4][5][6], thermal stability [7], and electrical conductivity which varies from metallic to semiconducting [8][9][10], transition-metal (TM) nitride thin films are used in a wide range of applications: from wear-resistant protective coatings for cutting tools and engine components [11; 12] to diffusion barriers in electronic devices [13]. The actual properties achieved by a given TM nitride film depend in large part on surface and microstructural evolution during reactive growth.…”

Ti-Al, Graded Al/AlTi, and Ti-Al-N Coatings Prepared by Supersonic Free-Jet PVD

“…[1][2][3] Thus, ductile bulk materials coated with heat-resistant and/or hard wear-resistant coatings have more potential than ordinary monolithic brittle materials. 4) Hard coatings consist of multiple layers to satisfy the requirements for strong adhesion to a substrate, high thermal stability, substantial hardness, low friction coefficient, and good compatibility. 5) This explains the necessity of functionally graded materials (FGMs) coating.…”