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

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.