J. Musil

Hard and superhard nanocomposite coatings

This article reviews the development of hard coatings from a titanium nitride film through superlattice coatings to nanocomposite coatings. Significant attention is devoted to hard and superhard single layer nanocomposite coatings. A strong correlation between the hardness and structure of nanocomposite coatings is discussed in detail. Trends in development of hard nanocomposite coatings are also outlined.

Relationships between hardness, Young's modulus and elastic recovery in hard nanocomposite coatings

Abstract The paper is devoted to an assessment of the mechanical behavior of hard and superhard nanocomposite coatings from loading/unloading curves measured by a computer-controlled Fischerscope H 100 microhardness tester and a maximum depth d max of the diamond indenter impression into the coating at a given load L . It is shown that: (1) the area between the loading/unloading curve and the value of d max decreases with increasing (i) hardness H , (ii) effective Youngs modulus E * = E /(1−ν 2 ) and (iii) universal hardness HU, where E and ν are the Youngs modulus and the Poisson ratio, respectively; and (2) there is no simple relation between the mechanical response of the coating and H or E * alone; however, this response is strongly dependent on the ratio H / E * . The last fact gives a possibility of tailoring the mechanical properties of a coating for a given application, e.g. to prepare coatings with high hardness H , high resistance to plastic deformation (∼ H 3 / E *2 ), high elastic recovery W e , but with low E * and high d max . Special attention is also given to the analysis of problems in accurately measuring the hardness of superhard (≥60 GPa) coatings. It is shown that a high elastic recovery W e ≥80% of superhard films with H ≥60 GPa (1) strongly decreases the gradient d H /d L and (2) shifts the region L , where H ( L )≈constant and the hardness H is correctly measured, to higher values of L . This means that the lowest load L used in the hardness measurement must be higher than L used in measurements of coatings with H H measured from being significantly higher than the real hardness of the coating.

Superhard nanocomposite Ti1-xAlxN films prepared by magnetron sputtering

Abstract Ti 1− x Al x N films were sputtered from an alloyed TiAl (60/40 at.%) target in Ar and Ar+N 2 mixture at a constant total pressure of 0.5 Pa by a planar round unbalanced magnetron of 100 mm in diameter. Films were sputtered at different partial pressures of nitrogen p N 2 ranging from 0 to 0.2 Pa, different substrate temperatures T s ranging from room temperature RT to 400°C and two substrate biases U s = U fl , i.e. floating potential, and U s =−200 V. It was found that: (i) the continuous change in p N 2 induces a dramatic change in the film structure and (ii) different values of microhardness of Ti 1− x Al x N films produced at different p N 2 , correlate well with changes in the film structure. Superhard (≥40 GPa) films with hardness of up to 47 GPa were prepared. The superhard films are nc-TiAlN/AlN nanocomposite films composed of relatively large (∼30 nm) TiAlN grains, oriented in one direction and surrounded by an amorphous and/or nc-AlN phase. These films exhibit a high elastic recovery up to 74% and contain about 20 at.% Ti, 25 at.% Al and 55 at.% N. The conditions under which superhard Ti 1− x Al x N films can be prepared are given.

Magnetron sputtering of hard nanocomposite coatings and their properties

The article reports on the present status of knowledge in the field of hard and superhard nanocomposite coatings prepared by magnetron sputtering. Special attention is devoted to the two-phase nanocomposites composed of one hard and one soft phase. Trends of the next development are outlined.

ZrN/Cu nanocomposite film : a novel superhard material

This article reports on the structure and hardness of ZrCu‐N films prepared by dc reactive magnetron sputtering of a ZrCu alloyed target in a mixture of Ar+N 2 using a round planar unbalanced magnetron of diameter 100 mm. It was found that there is a strong correlation between the structure of the film and its hardness. The hard (<40 GPa) ZrCu‐N films are characterized by many weak reflections from poly-oriented ZrN and Cu grains. In contrast, the superhard (µ40 GPa) ZrCu‐N films are characterized by a strong reflection from ZrN grains with a dominate ZrN(111) orientation and no reflections from Cu. The superhard ZrCu‐N films with a hardness of 54 GPa are nc-ZrN/Cu nanocomposite films composed of strongly oriented ZrN grains surrounded by a thin layer of Cu. These films exhibit a high elastic recovery of about 80% (determined by a microhardness tester) and contain approximately 1‐2 wt.% Cu. The superhard nc-ZrN/Cu nanocomposite films represent a new class of superhard materials of the type nc-MeN/metal.

A comparative study on reactive and non-reactive unbalanced magnetron sputter deposition of TiN coatings

Abstract Titanium nitride (TiN) coatings were deposited by unbalanced D.C. magnetron sputtering via the non-reactive and reactive technique using a TiN or Ti target, respectively. The differences of these sputter techniques have been studied in detail. Main emphasis was laid on the characterization of the ion bombardment parameters for both techniques. The ion energy and the ion/atom flux ratio was varied in the range between 30 and 120 eV and 0.1 and 10, respectively. Coating characterization was done with respect to morphology, chemical composition, crystallographic structure, hardness, and macrostresses during thermal cycling. The use of an ion energy of 30 eV combined with an ion/atom flux ratio of 8.6 and 10 results in a microhardness of approximately 47 GPa for non-reactive and reactive TiN coatings, respectively. Their biaxial stresses and grain sizes also show comparable values for both techniques of approximately −2 GPa and 23 nm, respectively. The similar properties of TiN coatings deposited using non-reactive or reactive sputtering are, however, only valid for an intense ion bombardment. The transition from porous columnar to dense fibrous structures requires a more pronounced activation of film growth by ion bombardment in the case of reactive deposition as compared to non-reactive sputtering. Mainly, this is a result of the higher energy of the N atoms and the three times higher deposition rate in the non-reactive process compared to the reactive one. Moreover, during reactive sputtering, energy is also needed to dissociate the molecular nitrogen gas. The results obtained should serve as a fundamental basis for the understanding of the differences in growth conditions for non-reactive and reactive sputter techniques. Furthermore, an explanation of the high hardness values of the coatings is given and the influence of thermal annealing on the defect density, grain size and microhardness of the coatings is presented and discussed in detail.

Structure and properties of hard and superhard Zr-Cu-N nanocomposite coatings

Zr‐Cu‐N nanocomposite films represent a new material of the type-nanocrystalline transition metal nitride (nc-MeN):metal. In the present work, films were deposited onto steel substrates using unbalanced dc reactive magnetron sputtering of a Zr‐Cu (62:38 at.%) target. Film structure, chemical composition, mechanical and optical properties were investigated by means of X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, wavelength dispersive electron probe microanalysis, depth-sensing microindentation and spectroscopic ellipsometry. It was found that (i) there is a strong correlation between the film structure, Cu content and film properties and (ii) either hard or superhard Zr‐Cu‐N films can be formed. The superhard coatings with hardness H\ 40 GPa are characterized by a columnar structure, a strong 111 XRD peak from ZrN grains and no diffraction peaks from Cu. These films exhibit a high elastic recovery of about 80% and contain a very low amount of Cu, approximately 1‐2 at.%. In contrast, the hard (B 40 GPa) Zr‐Cu‐N films are characterized by many diffraction peaks from polyoriented ZrN and Cu grains, a more random microstructure and a Cu content higher than 2 at.%. The optical properties of nanocomposite Zr‐Cu‐N films depend on the stoichiometry of the hard ZrNx compound and the content of Cu in the film.

Hard and superhard Zr–Ni–N nanocomposite films

This article reports on the relationship between the structure and mechanical properties of Zr–Ni–N nanocomposite films prepared by d.c.-reactive magnetron sputtering of a ZrNi (90/10 at.%) alloyed target in a mixture of Ar and N2 onto steel substrates using a planar round unbalanced magnetron with a diameter of 100 mm. The Zr–Ni–N nanocomposite films represent a new material of the type nc-MeN/metal composed of a hard nc-ZrN phase and a soft Ni phase, where nc- denotes the nanocrystalline phase of zirconium nitride. It was found that (i) there is a strong correlation between the structure of the film, the content of Ni in the nanocomposite and the film properties; (ii) Zr–Ni–N films can form superhard films with a high hardness up TO 57 GPa; and (iii) Zr–Ni–N films with the same hardness (H>40 GPa) can exhibit different structures and strongly differ in the size of nc-ZrNx grains of which they are composed. Therefore, the superhard coatings with a hardness of H>40 GPa are characterized either by the strong reflection from large (20–50 nm) ZrNx grains with a preferential orientation or by many weak reflections from small, approximately 10 nm, ZrNx grains. General relationships between the microhardness H, the reduced Youngs modulus E*=E/(1−ν2), and the elastic recovery We, determined from loading/unloading curves measured using a Fisherscope microhardness tester are given; here E and ν are the Youngs modulus and the Poissons ratio, respectively. The ratio H3/E*2, which represents the resistance of the material to plastic deformation, is also given.

Thermal stability of PVD hard coatings

Abstract Hard coatings deposited by physical vapour deposition based on the transition metal nitrides are nowadays widely applied to reduce tool wear. The aim of this paper is to show how microstructural parameters like grain size, stress and chemical and phase composition influence the thermal stability of different hard coatings. This is demonstrated using single-phase coatings like TiN, (Ti, Al, V)N and CrN as well as the dual-phase nanocomposite coatings CrN–Cr2N and TiN–TiB2. It is shown that the resistance against recovery and recrystallization can be improved by introducing a high density of phase boundaries, as is the case for nanocomposite coatings.

Hard nanocomposite Zr-Y-N coatings, correlation between hardness and structure

Abstract The article reports on structure and mechanical properties of Zr–Y–N nanocomposite films containing two immiscible elements (Zr, Y) as metals. Films were prepared by d.c.-reactive magnetron sputtering of alloyed targets ZrY (80/20 at.%) and ZrY (93/7 at.%) in a mixture of Ar+N 2 using round planar unbalanced magnetrons of diameter 100 mm. It was shown that: (i) there is a strong correlation between the structure of the film and its hardness, H ; (ii) the film structure can be controlled with the interlayer inserted between the substrate and the Zr–Y–N film; and (iii) the film hardness depends on the ratio N/(Zr+Y) in the film and the crystallographic orientation of ZrN grains. Superhard nanocomposite films with hardness greater than 40 GPa were prepared. These films are characterized by (i) the X-ray reflection from ZrN(200) grains and no reflection from the second phase containing Y; and (ii) the ratio N/(Zr+Y)≈1. Also, it was found that the incorporation of nitrogen into the pure ZrY alloy film results in dramatic changes of its mechanical properties. The Zr–Y–N film can be very hard (up to 47 GPa), exhibits the high elastic recovery ( W e up to 83%) and the high resistance to plastic deformation [ H 3 / E * 2 up to approx. 0.75, where E *= E /(1−ν 2 ), E is the Young’s modulus and ν is the Poisson’s ratio]. On the contrary, the ZrY alloy film is soft ( H ≈6 GPa) and exhibits a low elastic recovery ( W e =32%).