ICMCTF2013 Session F4-1: New Oxynitride Coatings

The oxygen containing coatings are well established products on the cutting tools market. Recently, commercially available PVD oxide coatings have been developed. In spite of growing importance of oxide containing wear protective coatings, there is only minor information available about their growth mechanism as well as their properties and cutting performance.

This paper gives an overview about the progress in the development of oxygen containing coatings using industrial scale PVD equipment. The advantages and the flexibility of the reactive arc evaporation for the coatings design are emphasized.

The deposition experiments were performed in a cathodic arc production system. The target composition was changed in order to influence the chemical composition of the coatings. Different coating stoichiometry was obtained through the variation of oxygen flow, while keeping nitrogen pressure constant. The correlation between the coating stoichiometry, phase formation and coating properties is presented. Finally examples for application of the oxyntiride coatings are given.

Injection and extrusion moulding is used to produce plastic parts for a wide variety of applications. In 2006 nearly 245,000 kt of plastics products were produced by means of extrusion and injection moulding. Due to rapidly increasing demands on individualized products, higher process stability and lower amounts of rejects new material concepts have to be developed. Especially, production of high precision lenses leads to high requirements for the production process. Every source of defects while processing has to be eliminated. For processing transparent polymers injection molding machines are most commonly used. While processing optical polymers like polycarbonate (PC) and polyether sulfone (PES) defects are produced by the plasticizing unit. Therefore, coatings for injection molding machines were developed by physical vapor deposition (PVD) in order to reduce the polymer defects as well as to protect the coated toll against wear and corrosion. All investigations were carried out on plasma nitrided ASTM A355 (X34CrAlNi7, 1.8550). TiN, (Ti,Al)N, (Ti,Al)ON, CrN, (Cr,Al)N and (Cr,Al)ON were deposited with Arc PVD. For all coatings a good adhesion to the substrate could be achieved. The adhesion affinity of the polymers to the coating surface was measured using high temperature contact angle measurements. With the PVD-coating (Cr,Al)ON the lowest adhesion of the polymers to the surface was achieved. This coating was applied on screw tips and tested in injection moulding experiments at different process parameters. After the tests, the produced parts were investigated regarding the defects on the polymer surface. The application of (Cr,Al)ON on the tools leads to reduction of defects on the polymer surface. The analysis of the coated parts showed that the tools could be cleaned much easier from the polymers after the experiments without delamination of the coating, so that the reduced adhesion was also confirmed in field tests.

Silicon (Si) and Yttrium (Y) are believed to have a strong influence on functional properties of hard thin films due to their effects in the growth process and incorporation in solid solution phases. It is worthwhile noting that even very low concentrations of these elements may be active for controlling structural evolution during the growth. The aim of this work is to systematically study the microstructural development and mechanical properties of (Al,Cr,Si/Y)O_xN_1-xoxynitride films as a function of Si and Y contents. Our results show that Si and Y have a different mechanism for incorporation into the oxynitride films. According to the TEM, XRD, and XPS results, substitutional role of yttrium is suggested for investigated coatings, whereas silicon tends to form an amorphous glassy phase. Such oxynitride layers meet an enhanced phase stability by the incorporation of both silicon and yttrium. As a result, nitride growth regime is extended in the range of O/(O+N) ≤ 0.80 with cubic (B1)-structured (Al,Cr,Si/Y)O_xN_1-x. However, on the other hand, Si and Y hinder the formation of corundum phase α-(Al,Cr,Si/Y)_2+δ(O_x,N_1-x)₃ in the predominant oxide regime (O/(O+N) ≥ 0.98) suggesting retarded diffusional processes with the presence of such minor alloying elements in the deposition flux. Additionally, the Y-containing oxynitrides demonstrate a greater nano-hardness for all growth regimes.

In literature, studies on phase formation of transition metal oxynitrides are rare. It has been proposed that the defect structure is important for NaCl structured CrAlSiNO and CrAlNO [1-4] and TiAlNO [5, 6]. While the formation of metal vacancies is commonly accepted for CrAlNO, it is not clear what defects occur in TiAlNO and if the properties are influenced.

Here, we study phase formation of TiAlNO by high power impulse magnetron sputtering in an industrial deposition system. With a N₂:O₂ flow ratio of 15:1, NaCl structured TiAlNO is detected with a (200) fibre texture exhibiting a lattice spacing of d₀₀₂ = 2.138 Å, which corresponds to a 0.4 % larger lattice spacing than TiAlN deposited under the same conditions, without intentional oxygen addition. The cause for the lattice spacing expansion will be discussed. Additionally, the influence of N₂:O₂ flow ratio on phase formation will be presented.

[1] Karimi A, Morstein M, Cselle T. Surf. Coat. Technol. 2010;204:2716.

[2] Najafi H, Karimi A, Dessarzin P, Morstein M. Thin Solid Films 2011;520:1597.

[3] Stueber M, Diechle D, Leiste H, Ulrich S. Thin Solid Films 2011;519:4025.

[4] Khatibi A, Sjölen J, Greczynski G, Jensen J, Eklund P, Hultman L. Acta Mater. 2012;in press.

[5] Sjölen J, Karlsson L, Braun S, Murdey R, Hörling A, Hultman L. Surface and Coatings Technology 2007;201:6392.

[6] to Baben M, Raumann L, Schneider JM. Journal of Physics D-Applied Physics 2012;accepted.

These applications cover mechanical, optical, electrical and chemical functionalities such as plasto-elastic properties, interfacial stress and adhesion, wear and tribological properties, refractive index and extinction coefficient, reflectivity and transmissivity, breakdown voltage and leakage current, corrosion protection and barrier features, wet and plasma etching behaviour as well as and environmental durability. Typical substrate materials for these applications are glass, ceramics, polymers, metals and steel.

In particular by means of PVD- and CVD-processes, both the stoichiometric silicon oxide (SiO₂) and the silicon nitride (Si₃N₄) can be deposited by fully reactive processes. For the majority of applications, it is very beneficial that almost all of the silicon oxinitrides (SiO_xN_y) can deposited within the same deposition chamber and with the same technology in a sufficiently stable way by partially bi-reactive processes. In principle, this allows a property tailoring from the pure oxide to the pure nitride mode with almost any intermediate state in between.

In addition to a short overview regarding the possible property range of mechanical, optical, electrical and chemical quantities, at least one application example of each major field of application is discussed in more detail. The interdependence of process characteristics, film properties and functional features is demonstrated. It has been shown that various plasma processes even at lower temperatures are available to meet the needs of advanced surface finishing. Moreover, vacuum-based plasma-processes provide cheap cleanroom-like conditions even under robust industrial environments.

In the current paper we have studied effect of different sputtering parameters such as nitrogen flow rate, deposition time and sputtering pressure on chromium oxynitride films. The X-ray diffraction studies show presence of Cr₂O₃ and Cr₂N phases with various textures depending upon the sputtering parameters. The stress are compressive and maximum stress (-4.5GPa) is observed for films deposited at 70sccm of nitrogen flow rate, the compressive stress (-0.2 to -5.3GPa) increases with deposition time but the stress is tensile (0.5 to 3.6GPa) and increases with sputtering pressure. The maximum contact angles observed are 102° for 70sccm nitrogen flow rate, 108.7° for deposition time of 140 minutes and 97.4° at sputtering pressure of 6Pa. The chromium oxynitride films are hydrophobic and can have potential applications as water repellent surfaces for self cleaning purpose. The transmission and absorption curves of chromium oxynitride films were recorded by UV-Vis-NIR spectrophotometer. The band gap of deposited chromium oxynitride films decreases with increase in nitrogen flow rate, deposition time while it increases with sputtering pressure.

Keywords: Chromium oxynitride, Sputtering, X-ray diffraction, Optical properties, Wettability