Formation Mechanisms and Structural Properties of Aluminium-Chromium Based Oxynitride Thin Films Synthesized by Physical Vapor Deposition

Oxynitride thin films, a novel class of mixed anionic solid structures, have gained an intense interest during recent years related to the unique interplay of oxygen (O) and nitrogen (N) with metals in the material structure. This matter can result in specific physical and structural properties. Accordingly, by varying the anionic composition (O/(O+N)), it is possible to tune the properties of oxynitrides, and particularly to enhance mechanical strength and phase stability. Consequently, the understanding of the role of O and N in such structures is of great interest. Taking this into account, we studied the formation mechanisms and properties of AlCr-based oxynitride films synthesized by the reactive magnetron sputtering (650◦C) and cathodic arc evaporation (550◦C) with an emphasis for application in hard coat- ings. Our results revealed that the variation of the anionic composition (O/(O+N)) disclosed the existence of three distinct growth regimes including: (i) a nitride regime O/(O+N)≤ 0.6, (ii) a transition regime 0.6 < O/(O+N) < 0.97, and (iii) an oxide regime O/(O+N)≥ 0.97, which were correlated to the structural features, chemical environ- ment, physical and mechanical properties of the films within each regime. For the oxygen fraction up to 60%, transmission electron microscopy (TEM) and x-ray diffrac- tion (XRD) alongside indicate the development of films in the nitride-type structure with a continues anionic substitutions, i.e. solid solution of AlCrOx N1−x . With fur- ther increase of the oxygen content, the structure of the layers is that of a poorly crystallized phase, and a near-amorphous film was favored in accordance with the observed low nanohardness, i.e. transition regime. Finally, occurring at the highest oxygen content, oxide regime (O/(O+N)≥ 0.97) was assigned to the films with oxide- type structure; however, with some probable oxygen substitution by nitrogen. The composition/structure variation was consistent with the chemical environment of the involved atoms, evaluated by x-ray photoelectron spectroscopy (XPS), where the electronic properties of the films exhibited substantial changes from regime to regime. Moreover, the film properties observed within each regime were also strongly in- fluenced by the cationic composition and deposition conditions. Al-rich layers (Cr/Al ≤ 0.1) were found to crystallize in the hexagonal wurtzite (B4) structure in the nitride regime (O/(O+N) ≤ 0.55) and transformed to a fully amorphous network and cubic γ-(Al,Cr)2O3 in the transition and oxide regimes, respectively. Conversely, Cr-rich films (Cr/Al≃1) were crystallized in the cubic (B1) structure within the nitride regime and preserved their structure in the transition regime despite the large pro- portion of oxygen. Such coatings in the oxide regime consisted of a solid solution of α-(Al,Cr)2(O0.97,N0.03)3 with corundum lattice and favorable protective proper- ties. Furthermore, it was observed that the structure of the films deposited at lower temperatures (400◦C) remains in the transition regime, and the oxide structure is not developed. The presence of silicon (Si, 3-4 at.%) and yttrium (Y≤ 1 at.%) in the oxynitride films was found to change the thresholds and structural features within the growth regimes. Oxynitride layers exhibited an enhanced phase stability (i.e. cubic-B1) by the incorporation of both Si and Y. As a result, the nitride regime was extended in the range of O/(O+N)≤ 0.8. However, on the other hand, Si and Y prohibit the formation of corundum phase in the oxide regime suggesting retarded diffusional processes with the presence of such alloying elements. Considering the preparation conditions of the oxide coatings, we found a previously unobserved formation of cubic crystalline fcc-(Al,Cr)2O3 and its transition to the well-known corundum phase (α- (Al,Cr)2O3) at higher film thickness beyond 2 μm by employing an extra-high substrate bias during arc evaporation. The TEM, XRD, and XPS results alongside indicated that the cubic (B1) layer acquires the properties of a metastable monoxide (M1−x O), which is kinetically stabilized by the occurrence of vacancies. It was proposed that there is a thickness-dependent gradient in point defect density, and its modification leads to the destabilized fcc-(Al,Cr)2O3 and a cubic-to-corundum transformation occurs. Favorable phase stability and oxidation resistance together with the low wear rates and friction behavior observed for the oxynitride films underline their potential for hard coating of tools used under severe dry cutting conditions.

Optimization of composition and structure of cemented carbide cutting tools

Samy Adjam

WC-Co cemented carbides are composites, which combine a hard phase consisting of WC grains and a metallic ductile phase as a binder. Their excellent mechanical properties, combining high hardness, toughness and refractory properties, make them excellent materials for cutting processes such as turning, drilling and milling. For such applications, cemented carbides are coated with thin ceramic films that increase their resistance to thermal stress as well as to the mechanical and chemical wear inherent in extreme machining conditions. Three main characteristics determine the performance and wear resistance of cutting tools: the cohesion of the hard phase above 1200 K, the ductility of the cobalt binder, and the mechanical and thermal resistance of the coatings. This work focuses on the impact of the microstructural behaviour of the cobalt binder on the tool-life of the cutting tools. The aim is to develop new binders for cemented carbides with improved impact resistance. The techniques of transmission electron microscopy at room temperature and in-situ at high temperature, as well as mechanical spectroscopy in a forced torsion pendulum have proved to be essential for the identification of the microstructure of cobalt and for the study of the relaxation phenomena occurring therein. Mechanical spectroscopy has evidenced the presence of two relaxation peaks, P1 and P2 in the cobalt phase and a peak (P3) related to WC-WC grain boundary sliding. The peak P1 appears to be correlated with the cutting tool life measured in interrupted cutting tests. In this work, the role of the binder is highlighted in particular by the identification of a glass-type transition observed for cobalt. The thermodynamics of the transition is evidenced by the behavior of P1 whose relaxation time diverges according to a Vogel-Fulcher-Tammann model, which is characteristic of glass transitions. The state of the material corresponding to temperatures below this transition shows a strong dependence on its thermomechanical history. This dependence is confirmed by a divergence in plastic deformations over two identical temperature cycles with different thermomechanical histories. Such a divergence on a standard protocol, called (ZFC/FC), seems to indicate a break in the thermodynamic hypothesis of ergodicity, even though it could not be attributed solely to the behaviour of cobalt. Transmission electron microscopy at room temperature has determined that the low temperature phase of the cobalt binder has a face-centred cubic structure and is composed of twinned nanodomains containing nanotwins. This state is therefore characterised by a short-range order without the long-range order, which is characteristic of glassy materials. In-situ observations have highlighted a detwinning process corresponding to the temperatures of the cobalt relaxation peak identified by mechanical spectroscopy. This detwinning process continues until the appearance of a long-range ordered face-centred cubic phase above 1050K, which is favourable to deformation. Mechanically, this transition is similar to a brittle-to-ductile transition, greatly influencing the properties of the material and its toughness. The cutting tool life could benefit from lowering the brittle to ductile transition temperature.

EPFL2021

Formation of cubic structured (Al1-xCrx)(2+delta)O-3 and its dynamic transition to corundum phase during cathodic arc evaporation

Alireza Karimi, Hossein Najafi

Mixed aluminum-chromium oxide coatings in the form of (Al,Cr)(2)O-3 solid solutions have attracted extensive research interest during the past years due to their successful use for challenging wear applications and the comparative ease of their moderate-temperature deposition by physical vapor deposition techniques. During our research into the reactive cathodic arc deposition of this type of coating, we found a previously unobserved transition between two crystalline aluminum-chromium oxide structures. During the early growth stage, films arc-deposited from Al10.55Cr0.45 targets form a first zone, that was found to contain exclusively the metastable cubic fcc-(Al1-xCrx)(2+delta)O-3 phase. This kinetically favored phase is reproducibly followed by the growth of a second zone made of the initially expected corundum phase, alpha-(Al1-xCrx)(2+delta)O-3, as observed by TEM. This dynamic transition has a significant effect on the film properties. XPS studies and structural data show that the formation of fcc-(Al1-xCrx)(2+delta)O-3 with a (200) preferred orientation arises from the initial presence of a metastable monoxide (M1-xO) film, which is stabilized by the incorporation of metal vacancies (31%) in the B1 structure. However, as the thickness of coating increases, the thermodynamic aspect becomes more important as compared to kinetics and leads to a loss of structural stability in the cubic layer, which is a kinetically favored phase. As a result, the system will transform into the metastable corundum alpha-(Al1-xCrx)(2+delta)O-3, which is thermodynamically more stable than the cubic phase. In this paper, formation of fcc-(Al1-xCrx)(2+delta)O-3 and its transformation to corundum phase are discussed in detail with respect to the structural and electronic properties of the different phases. (C) 2012 Elsevier B.V. All rights reserved.

Elsevier Science Sa2013

Structure and electronic properties of AlCrOxN1-x thin films deposited by reactive magnetron sputtering

Alireza Karimi, Hossein Najafi, Emadeddin Oveisi

In this study, the main attempt is devoted to investigating the microstructure and electronic properties of AlCrOxN1 - x films in a wide range of oxygen concentrations from 0 to 1. These oxynitride films were deposited by pulsed DC magnetron sputtering from Al55Cr45 targets. Our results showed that films with x = O/(O + N) < 0.6, exhibit a cubic (B1) lattice with a well-developed columnar structure. The incorporation of oxygen into the films without any oxide segregation results in the formation of a substitutional AlCrOxN1 - x solid solution and material system behaves like nitrides electronically. In the range of oxygen contents from 0.6