ICMCTF2014 Session B5-1: Hard and Multifunctional Nano-Structured Coatings

Alloying with transition metal (TM) elements into Ti-Al-N to improve its performance is attracting considerable interest. Here, we investigate the effect of Hf on structure, mechanical and thermal properties of Ti-Al-N coating with cubic structure. Alloying with 3 at.% Hf slightly promotes the spinodal decomposition of Ti-Al-N to form Al-depleted and Al-enriched domains during thermal annealing. According to TGA and DSC results, Hf-containing coating exhibits less oxidation below 1053 °C, which is attributed to the retarded transformation of anatase (a) TiO₂ to rutile (r) TiO₂. However, incorporation of Hf results in worse oxidation resistance above 1053 °C due to the deterioration of barrier effect of diffusion from large Hf atom. Ti-Al-Hf-N coatings onto Al₂O₃ substrates can retain most of dense unoxidized layer after oxidation at 850 °C for 10 h under synthetic air atmosphere and fully transform to oxides when oxidation temperature elevate to 950 °C. An oxidation of the coatings following the DSC curve during air up to 1100 °C shows that the oxide scales of Ti-Al-N and Ti-Al-Hf-N coatings are 3.45 and 3.87 μm, respectively.

The financial support from National Natural Science Foundation of China (grant nos. 51371201 and 51371199) and Zhuzhou cemented carbide cutting tools limited company of China is acknowledged.

Transition metal nitrides are used in many applications because of their high hardness, mechanical wear resistance, high thermal stability, and good oxidation resistance. For these materials, much research has concerned nanocrystalline films and nanocomposites. Especially in the development of hard ceramic coatings for wear-resistant applications, microstructural design has been of great importance, as has the correlation between microstructure and mechanical properties. Recently, we have directed focus to a less studied area: amorphous transition metal nitrides. We propose amorphous multicomponent transition metal nitrides for a new class of durable materials that could extend the range of possible applications, due to the material’s homogeneous structure, lack of weak grain boundaries, and mixed character of bonding. We previously showed that the addition of Al and Si to TiN in Ti_1-x-yAl_xSi_yN distorts nanocrystalline growth and promotes renucleation and an amorphous structure in cathodic arc evaporation [1]. We concluded that the coatings were amorphous when the Ti content 1-x-y<0.34. Isothermal annealing experiments showed that the amorphous coatings were thermally stable up to 900 °C and exhibited age hardening up to 1100°C with an increase in hardness from 19.4 GPa to 31.6 GPa. Here, we exchange Al for B and explore the Ti_1-x-yB_xSi_yN system. B is a well-known grain refiner, and a good choice for promoting growth of amorphous phases, since it can be both three- and fourfold coordinated. Also, B-N bonds are stronger than Si-N ones, which is beneficial for hard coating applications. The coatings were grown onto cemented carbide substrates in an industrial scale cathodic arc evaporation system using Ti-Si and Ti-B cathodes in a pure N₂ atmosphere. Compositional analysis of the as-deposited films was performed by elastic recoil detection analysis, and the structural information and phase composition was gained by X-ray diffraction, and analytical transmission electron microscopy. Mechanical properties of the coatings are characterized by nanoindentation. We show that the structure of as-deposited films remain amorphous up to a Ti content of 0.63. The structure changes to nanocrystalline for Ti content 1-x-y>0.70. The hardness of the as-deposited amorphous coatings is 17.1-18.9 GPa depending on composition. In addition, we will present results for the phase composition, thermal stability, and crystallization behavior of the coatings, as well as from metal cutting tests.

[1] H. Fager et al., Surf. Coat. Technol. (2013), 10.1016/j.surfcoat.2013.07.014

CrTiAlN coatings with different Cr/Ti ratios have been reactive sputtered from a CrAl and a Ti target, which were powered by pulsed dc magnetron sputtering and deep oscillation high power pulsed magnetron sputtering (DOMS), respectively. DOMS is an alternative high power pulsed magnetron sputtering technique that uses large voltage oscillation packet to achieve high power pulses for sputtering. The coatings were deposited onto WC-Co, AISI 304 steel coupons, and Si wafers at room temperature using a -60 V dc negative substrate bias voltage. The microstructure, mechanical properties, and oxidation and tribological behavior at elevated temperatures of the CrTiAlN coatings were investigated and compared to the Cr_0.7Al_0.3N baseline system.

Icosahedral boron-rich solids, containing B₁₂ clusters, exhibit outstanding physical properties¹ such as high hardness, a Young’s modulus above 500 GPa² and a melting point on the order of 2400 °C. Calculations^3,4 and experiments⁵ have shown that these materials provide very favorable properties for wear resistant applications. Furthermore, spontaneous self-healing¹ has been reported after radiation induce vacancy formation. Among others AlYB₁₄ (space group: Imma) belongs to this group of boron-rich solids. However, synthesizing crystalline XYB₁₄, where X and Y are metals, is challenging both with respect to the incorporation of impurities⁵ and the required synthesis temperature: 1400 °C.⁶

The influence of the duty cycle t_on/(t_on+t_off) on the structure evolution of AlYB₁₄ was studied using high power magnetron sputtering (HPPMS). The film structure was analyzed by X-ray and electron diffraction. Depending on the duty cycle the formation of crystalline AlYB₁₄ was obtained at a temperature of 800 °C. Our data indicate that crystalline AlYB₁₄ grains grow within an amorphous matrix.

References:

¹ D. Emin, J. Solid State Chem. 179 (9), 2791 (2006).

² J. Emmerlich, N. Thieme, M. to Baben, D. Music, and J. M. Schneider, J. Phys.: Condens. Matter 25 (33), 335501 (2013).

³ J. E. Lowther, Physica B 222 (2002).

⁴ Y. Lee and B. N. Harmon, J. Alloys Compd. 338 (2002).

⁵ B. A. Cook, J. L. Harringa, T. L. Lewis, and A. M. Russell, Scripta Mater. 42 (2000).

⁶ M. M. Korsukova, T. Lundström, and L.-E. Tergenius, J. Alloys Compd. 187 (1992).