ICMCTF2019 Session B4-3-WeM: Properties and Characterization of Hard Coatings and Surfaces III

Keywords: Hard Coating, Chromium nitride, Dynamic Glancing angle deposition

The addition of aluminum nitride to titanium nitride is known to improve the high-temperature oxidation resistance of TiN-based coatings. Furthermore, low or medium concentrations of Al increase also the hardness of the (Ti,Al)N coatings [1]. High Al contents lead typically to an enhanced decomposition of titanium aluminum nitride into Ti-rich (Ti,Al)N with the face centered cubic (fcc) structure and wurtzitic AlN, and to the degradation of the mechanical properties of the Al-rich (Al,Ti)N coatings. In order to be able to achieve good high-temperature oxidation resistance and high hardness concurrently, metastable Al-rich fcc-(Al,Ti)N must be stabilized.

In this comparative study, the mechanisms stabilizing metastable Al-rich fcc-(Al,Ti)N are discussed and tested on the Al-rich (Al,Ti)N coatings containing more than 50 mol % AlN, which were deposited using chemical vapor deposition (CVD), spotless arc evaporation (SAD) and pulsed magnetron sputtering (PMS). The mechanisms stabilizing the thermodynamically metastable phases were concluded from the microstructure analyses that were performed using X-ray diffraction, transmission electron microscopy with high resolution, X-ray spectroscopy (EDX) and electron-energy loss spectroscopy (EELS).

In the (Al,Ti)N coatings prepared by CVD, three-phase composites consisting of fcc-(Ti,Al)N, fcc-(Al,Ti)N and w-AlN have formed. Although w-AlN is known to deteriorate the mechanical properties of the (Al,Ti)N coatings, the interaction of these phases retarded the decline of the hardness up to x_AlN ~ 0.9. In the (Al,Ti)N coatings deposited by PMS, the formation of w-AlN was avoided up to x_AlN ~ 0.65. The Al-rich fcc-(Al,Ti)N phase was apparently stabilized by fluctuations of the Al (and Ti) concentration. The concentration of Al (and Ti) showed almost bimodal distribution in these coatings. In the (Al,Ti)N coatings deposited using SAD, the Al-rich fcc-(Al,Ti)N was stabilized mainly by a low ad-atom mobility, which promoted the formation of supersaturated fcc-(Al,Ti)N.

[1]D. Rafaja, C. Wüstefeld, M. Dopita, V. Klemm, D. Heger, G. Schreiber, M. Šíma, Surf. Coat. Technol. 203 (2008) 572-578]

Keywords: metastable fcc-(Al,Ti)N, microstructure, chemical vapor deposition, spotless arc evaporation, pulsed magnetron sputtering, XRD, TEM

Al containing transition metal nitrides undergo a characteristic phase transition from cubic B1 to hexagonal B4 structure as the Al content is increased. This change in crystal structure is known to affect not only mechanical property, but also chemical property such as oxidation resistance [1]. AlCrN and AlTiN are one of compounds among such system and applied tribological components such as cutting tool, die and molds for superior mechanical and oxidation resistance at elevated temperatures. A series of AlCrN and AlTiN coating with different Al contents were deposited by cathodic arc ion plating with different substrate bias voltage. Crystal structure of the coatings were examined by XRD and oxidation behavior was investigated by annealing samples in air at 800, 900 and 1000 °C for 30min and surface O composition was measured by EDX and AES for O depth profiling.

Both AlCrN and AlTiN coating showed change in crystal structure depending on the substrate bias, especially Al content is more than 70 at% for AlCrN and 50 at% for AlTiN. These coatings contain hexagonal phase in the coating at low substrate biases and became cubic single phase as the substrate bias is increased. In case of AlCrN, the oxidation behavior seems influenced by the crystal structure and much better oxidation resistance was observed for cubic single phase coating independent of Al content. In case of AlTiN, however, such crystal structure dependent oxidation behavior was not observed. X-ray diffraction analysis of these AlCrN and AlTiN coatings suggests systematic change in grain size corresponding to the change in substrate bias as indicated by width of the diffraction peak. From this observation, it can be concluded that change in oxidation resistance is more likely correlating to the change in grain size of the coating rather than crystal structure.

References:

[1] Reiter et al. Surf. Coat. Technol., Vol. 200, (2005) 2114

The purpose of this study were to investigate the toughening mechanism of V_1-xMo_xN nanocrystalline thin film with different composition of Mo, and compare the texture, residual stress and fracture toughness with each specimen. It is commonly acknowledged that nanocrystalline ceramics with high strength and hardness, while always show low ductility, which limits their applications. Recently, Sangiovanni et al. [1] predicted the V_0.5Mo_0.5N coating with high hardness and ductility by ab initio density functional theory (DFT), and it has been verified by experiments that V_0.5Mo_0.5N is more ductile than VN. However, there is lack of studies on the accurate stress and fracture toughness measurements of V_1-xMo_xN and the associated fracture mechanisms. Also it is important to understand the toughness enhancing mechanisms compared with different compositions of V_1-xMo_xN. Therefore, this study aimed to investigate the relationship of texture, residual stress and toughness of the different compositions of V_1-xMo_xN. V_1-xMo_xN thin films about 1000 nm were deposited on Si substrate by unbalanced magnetron sputtering (UBMS), the compositions of Mo were set to 0.1, 0.2, 0.3, 0.4 and 0.5 by adjusting Mo target currents. After deposition, the ratio of V/Mo and (V+Mo)/N were determined by X-ray photoelectron spectroscopy (XPS), the thickness of all specimens were confirmed by auger electron spectroscopy (AES) and scanning electron microscope (SEM). X-ray diffraction (XRD) was used to characterize the structure and the texture. Besides, the residual stress of the specimens was measured by laser curvature method (LCM) and average X-ray strain (AXS) combined with elastic constant from nanoindentation [2], the hardness was assessed by nanoindentation, and the internal energy induced cracking (IEIC) method was used to evaluate the fracture toughness.

[1] D. G. Sangiovanni, V. Chirita, L. Hultman, Phys. Rev. B, 81 (2010) 104107.

[2] A.-N. Wang, C.-P Chuang, G.-P. Yu, J.-H. Huang, Surf. Coat. Technol., 262 (2015) 40.

CrAlMoSiN nanocomposite coatings were produced by doping Mo into the CrAlSiN nanocomposite matrix via radio frequency magnetron sputtering. CrAlMoSiN thin films with different Mo contents were deposited on both Si-wafer and Inconel-718 substrate by controlling the Mo target working power. Since CrAlSiN nanocomposite coatings exhibit superior high temperature wear resistance, the addition of Mo into CrAlSiN nitride coatings will offer extra self-lubricating characteristic, which leads to lower friction coefficient. By doping Mo in to CrAlSiN coating via composition control, CrAlMoSiN coatings with high temperature wear resistance and reduced friction coefficient could be developed.

The chemical compositions of as-deposited coatings were identified by a FE-EPMA. The tribological property was evaluated by a tribometer with temperature control unit. The wear tracks were analyzed by an Alpha-step to calculate the wear rate. The nano-hardness (HIT) and reduced elastic modulus (EIT*) were examined by a nano-indenter. Further, microstructure of wear tracks was analyzed by FE-SEM and HR-TEM. The phase transformations were observed by a Gazing Incidence XRD and XPS.

Monolithic and multilayered W–N and W–Si–N coatings were fabricated through direct current magnetron cosputtering with a nitrogen flow ratio (N₂/(N₂ + Ar)) of 0.4 at substrate holder rotation speeds of 0 and 5 rpm, respectively. The characteristics and oxidation behaviors of the W–N and W–Si–N coatings were investigated by nanoindentation technique, X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. The mechanical properties of crystalline W–N coatings correlated to their texture and residual stress. The monolithic W₇₇N₂₃ samples located nearest to the W target exhibited a high deposition rate of 18.0 nm/min, a strong (200) texture coefficient, a high nanoindentation hardness of 32.7 GPa, a high Young’s modulus of 392 GPa and a residual stress of −3.2 GPa. The addition of Si into the W–N matrix transformed the coatings to be an X-ray amorphous phase dominated structure comprising constitutions of Si₃N₄, W₂N, and W. The preferential formation of Si₃N₄ declined the residual stress and mechanical properties of the W–Si–N coatings with increasing the Si contents. By contrast, the oxidation resistance was improved by adding a Si content > 24 at.% after annealing at 600 ◦C in a 1% O₂–99% Ar atmosphere.

The TaSiN nanocomposite thin films were fabricated by a reactive radio frequency ,r.f., magnetron sputtering system with pure Ta and Si sources. The Ar/N₂ flow ratio was fixed at 18/2 sccm/sccm, while r.f. input powers for Ta and Si were from 50 to 200W and from 50 to 150W, respectively. The Si contents doped in the coating ranged from 0 to approximately 25 at.%. The plasma of various process conditions were investigated by Optical Emission Spectroscopy.Characterizations by XRD, TEM, SEM revealed the dependence of Si dopeing and SiN_x phases on the preferred orientation, crystalline behavior, microstructure. Mechanical properties through wear tester, Rockwell.C and nano-indentation were evaluated to check the durability, adhesion and hardness, respectively. When the content of silicon reaches 17 at.%, the TaSiN structure evolved from crystallization to amorphous, leading to a significant degradation of mechanical properties. The TaSiN with lower Si contents exhibited TaN with SiN_x phase and processed superior mechanical strength.