The article reports on the enhanced hardness of nanocomposite coatings, their thermal stability, protection of the substrate against oxidation at temperatures above 1000 °C, X-ray amorphous coatings thermally stable above 1000 °C and new advanced hard nanocomposite coatings with enhanced toughness which exhibit (i) low values of the effective Young's modulus E ⁎ satisfying the condition H/E ⁎  > 0.1, (ii) high elastic recovery W e  ≥ 60%, (iii) strongly improved tribological properties, and (iv) enhanced resistance to cracking; here E ⁎  = E(1−ν 2 ), E is the Young's modulus and ν is the Poison's ratio. At the end trends of next development of hard nanocomposite coatings are briefly outlined.

Hard nanocomposite coatings: Thermal stability, oxidation resistance and toughness

Perspectives on oblique angle deposition of thin films: From fundamentals to devices

Provided is a surface-coated cutting tool combining superior heat resistance, superior wear resistance, and superior lubricity. A surface-coated cutting tool of the present invention includes a substrate and a coating formed on the substrate, and the coating is characterized in that the coating is formed by physical vapor deposition and includes one or more layers, that at least one of the one or more layers is a first coating layer, and that the first coating layer contains aluminum and nitrogen, has a thermal effusivity of 2,000 to 5,000 J·sec −1/2 ·m −2 ·K −1 , has a thickness of 0.2 to 5 μm, and has a crystal structure including a hexagonal structure.

Surface coated cutting tool

Abrasive wear behaviour of conventional and nanocomposite HVOF-sprayed WC–Co coatings

At present, there is neither standard test procedure nor standard methodology for assessment of toughness of thin films. However, researchers have long been trying to make such measurements, thus a spectrum of test methods have been developed, mostly each in its own way. As qualitative or semiquantitative assessment, a simple plasticity measurement or scratch adhesion test can mostly suffice. For quantitative description, however, a choice of bending, buckling, indentation, scratching, or tensile test has to be made. These testing methods are either stress-based or energy-based. This paper gives a critical review on these methods and concludes that, for thin films, the energy-based approach, especially the one independent of substrate, is more advantageous.

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Toughness measurement of thin films: a critical review

The microstructures of two tungsten carbide–cobalt (WC–Co) coatings, deposited using high velocity oxy-fuel (HVOF) thermal spraying method in different conditions, are studied. They are compared with that of the WC–Co powder grains injected in the flame, in an attempt to understand the transformations that occur during deposition. For this purpose, various imaging and analytical techniques in electron microscopy are used, in addition to global characterization methods such as X-ray diffraction and fluorescence. These methods reveal that the coatings are made of distinct islands, elongated along the substrate direction, which exhibit a nano-crystalline matrix containing tungsten, cobalt and carbon. The fraction of WC grains in the coating is smaller than that in the powder and fluctuates throughout the coating. A net loss in carbon is evidenced in the coatings as compared to the powder grains. New phases, W2C and W, appear in specific locations in the microstructure in relation with the local composition of the matrix. Very little metallic cobalt is retained. The extent of the transformation is related to the spraying conditions. Some processes that account for the change in microstructure and composition during spraying are proposed.

A study of high velocity oxy-fuel thermally sprayed tungsten carbide based coatings. Part 1: Microstructures

Sand erosion tests were performed on WC-Co and WC-CoCr coatings deposited by the high velocity oxy-fuel spraying method. Several analytical techniques, including X-ray diffraction, Auger electron spectroscopy and energy-dispersive spectroscopy in a transmission electron microscope were used to characterize the microstructures formed during powder processing and spraying. It was found that a substantial fraction of WC decomposed into W2C or reacted with the cobalt matrix to form ternary carbides such as Co3W3C and other mixed compounds. In both cases the binder phase had a nanocrystalline structure of size 4-8 nm containing tungsten, cobalt, carbon and chromium elements. The addition of chromium inhibits to a large extent the decomposition of WC and avoids the formation of metallic tungsten. In addition, chromium improved the erosion resistance by several times compared with the WC-Co coating. Scanning electron microscopy showed that the CoCr matrix binds carbides better than the cobalt matrix, thereby inhibiting carbide loss at the spray particle boundaries. The hydroabrasive wear behaviour of coatings and the mechanisms for material removal are discussed with respect to the microstructures formed during spraying.

Microstructure and hydroabrasive wear behaviour of high velocity oxy-fuel thermally sprayed WCCo(Cr) coatings

We have investigated the structural and mechanical properties of polycrystalline Cr–Si–N thin films of different morphologies. The films were deposited by reactive magnetron sputtering at 510 K using two confocal targets of Cr and Si. The structure and mechanical properties of co-deposited Cr–Si–N films depend on their silicon content. Films with silicon content lower than 3 at.% are fcc Cr 1− x Si x N compounds with a maximum hardness of 22 GPa. On the contrary, films containing more than 3 at.% of silicon show the segregation of a SiN x amorphous phase and highly columnar morphology. These samples have low hardness values (14 GPa). The alternating deposition of CrN and a-SiN x layers disrupts this columnar structure and leads to nanocomposite films. The hardness of these films varies when the multilayer period is changed, i.e. the CrN crystallite size. A maximum hardness value (26 GPa) has been found for a nanocomposite structure composed of 4 nm CrN crystallites embedded in amorphous SiN x .

Mechanical properties of nanocomposite and multilayered Cr-Si-N sputtered thin films

Niobium nitride NbN x films with 0.61⩽ x ⩽1.06 were prepared by reactive magnetron sputtering of a Nb metal target in an Ar/N 2 atmosphere at a total pressure of 0.4 Pa. Different nitrogen partial pressures P N 2 and substrate temperatures T s were used. The films were characterized by X-ray diffraction, electron probe microanalysis, scanning electron microscopy and nano-indentation measurements. The films deposited without substrate heating contain a pure cubic δ-NbN phase for 0.61⩽ x x ⩽1.06, they include a mixture of hexagonal NbN and cubic δ-NbN phases. The hardness variation correlates with changes in grain size and film morphology. Compact poorly crystallized films exhibit high nanohardness values of approximately 32 GPa. For films deposited at various P N 2 with heating the substrate to 400 °C, the crystalline structure was mixed (δ′-NbN+δ-NbN) phases. The concentration of the hexagonal δ′-NbN phase in the film changes with x . The hardness and the Young modulus follow the variation of the concentration of the hexagonal δ′-NbN phase. The films with high concentration of the hexagonal δ′ phase exhibit high hardness and Young modulus. It appears that the hardness of the hexagonal δ′-NbN phase is 40 GPa, whereas the hardness of the cubic δ-NbN phase is 25 GPa.

Structural and mechanical properties of sputtered cubic and hexagonal NbNx thin films

The effects of bilayer thickness and chromium content on the microstructures and mechanical properties of nanolayered TiAlCrN thin films were studied. The films were deposited onto the WC–Co substrates using cathodic arc method where 4 TiAl alloyed targets and 2 Cr targets were mounted alternately. Complex multilayers were produced by rotation of the sample holder and by controlling deposition conditions. Addition of chromium favors the formation of columnar structures in TiAlCrN layers. In thick bilayer films (Λ=400 nm) the TiAlN layer periodically interrupts the formation of columns in TiAlCrN, while for thinner bilayer films (Λ=10 nm) the columns are not interrupted leading to the formation of perfectly columnar films. Both Cr content and bilayer thickness contribute to hardness enhancement. The addition of Cr favored the formation of the harder cubic (Cr,Al)N phase rather than the softer wurtzite, hcp-(Al,Ti)N phase. The hardness enhancement due to multilayering can be explained by two factors; a grain refinement that leads to an increase in hardness and the formation of columnar structures with a preferred orientation. Indentation induced crack morphology appeared very sensitive to film microstructures, where well organized straight through thickness cracks occurred while indentation of TiAlCrN caused a network of irregular cracks due to the greater contribution of column grain boundary sliding.

Ayatollah Karimi

Papers

A study of high velocity oxy-fuel thermally sprayed tungsten carbide based coatings. Part 1: Microstructures

Microstructure and hydroabrasive wear behaviour of high velocity oxy-fuel thermally sprayed WCCo(Cr) coatings

Mechanical properties of nanocomposite and multilayered Cr-Si-N sputtered thin films

Structural and mechanical properties of sputtered cubic and hexagonal NbNx thin films

Microstructure and mechanical behavior of TiAlCrN multilayer thin films