F. Löffler

Process and advantage of multicomponent and multilayer PVD coatings

Abstract This paper gives a short review of physical vapour deposition (PVD) processes suitable for the deposition of wear-protective films with complex compositions and structures. It then characterizes the advantages of multicomponent and multilayer coatings, and describes their behaviour in cutting tool applications—an area in which PVD coatings are widely used in industry. The research results presented indicate a great future application potential for PVD coatings with complex compositions and structures. Particularly promising in this respect are films which achieve significantly prologned tool lives in interrupted-cut machining, such as Ti-Zr-N, Ti-C-N and TiN/Ti-C-N multilayer coatings.

SUPERHARD Ti–B–C–N COATINGS

Abstract In order to advance the development of extremely hard coatings, new homogeneous and metastable films within the Ti-B-C-N system were prepared by magnetron sputter ion plating using a TiB 2 target in the d.c. mode. Multicomponent layers of different compositions and structures were deposited by varying the reactive gases methane (CH 4 ) and nitrogen (N 2 ) and the dominant process parameters ( e.g. sputtering power and substrate bias). X-ray diffraction studies of the coatings revealed either an amorphous structure or crystallization in a hexagonal lattice. The lattice parameters were strongly influenced by the quantity of carbon and nitrogen incorporated. Electron microprobe analysis was used to determine the coating compositions. Evaluation of the characteristic light element X-ray spectra provided initial qualitative data on chemical bonding properties. Auger electron spectroscopy confirmed the construction of the films. The Palmquist method and hertzian indentations were used to characterize film toughness. The remaining mechanical coating properties were investigated by means of microhardness measurements and the CSEM scratch test. Scanning electron microscopy indicated an extremely dense coating structure. Superhardness up to values of 7000 HV 0.05 was the dominant property. Coating thicknesses ranging from 6 to 25 μm were realized. Behaviour of the Ti-B-C-N layers in abrasive wear tests was favourable.

Diffusion barrier design against rapid interdiffusion of MCrA1Y and Ni-base material

Abstract A physical vapour deposition, diffusion barrier coating in the system A1-O-N offers the possibility to reduce the interdiffusion between the layers of MCrA1Y and the Ni-base material. The barrier function depends on the high temperature stabilization of the amorphous structure. While amorphous A1-O undergoes modification at 1100°C from several crystalline substructures up to the ϵ phase, A1-O-N remains in the amorphous structure at 1100°C. This coating acts successfully as a passive diffusion barrier, and was tested up to 400 h at 1100°C in annealing tests and up to 2500 cycles in a thermal fatigue test rig with 1115°C and 200°C as the high and low peak temperatures. The gradient-free transition step achieved in the element analysis of the depth profiles, together with the inspection of the cross-sections, confirm the excellent barrier performance.

Deposition, properties and performance behaviour of carbide and carbonitride PVD coatings

Abstract In recent years, significant improvements in the performance and quality of many machining processes have been achieved through physical vapour deposition hard coating of high speed steels and carbides. Under machining parameters with low machining process temperatures, e.g. in interrupted cutting, the Ti-C-N coating has proved particularly effective, firmly establishing itself in the market. However, industrial coating practice reveals that a variety of problems with the process and materials occur during deposition of Ti-C-N. Process instabilities are observed and the carbonitride films tend to spall. Coating properties also deteriorate markedly at low coating temperatures. A comprehensive study of this complex problem has indicated that the properties of the carbon-carrier gas used in the process greatly affect the process behaviour and coating properties. The paper presents the results of experimental work with four carbon-carrier gases. Ethane is characterized by its balanced reaction behaviour, as compared with acetylene, ethylene and methane.

PVD coatings for diecasting moulds

Abstract In the field of non-ferrous diecasting, the tool life of the diecasting mould critically affects the cost effectiveness and quality standard of the foundry. It is therefore desirable to prolong the tool lives of diecasting moulds. The primary stresses to which the surface of the mould is exposed are thermal shock, abrasion and chemical reactions between the liquid aluminium and the iron in the steel mould. Certain regions of the mould are subjected to especially high stresses. In particular, these include ribs and the near-gate sections, where abrasion occurs owing to the high velocity of the melt. In the past a number of practice test have been made with PVD-coated diecasting moulds. TiN was generally used as the protective coating material. These tests failed to achieve satisfactory results, i.e. the tool life was not significantly improved. The paper presents results obtained with Cr-based PVD coatings, which led to substantially increased tool lives.

Arc-deposited Ti-Zr-N coatings on cemented carbides for use in interrupted cutting

Abstract Hard coatings can significantly improve the performance of cemented carbide tools. In the case of carbides coated using chemical vapour deposition (CVD), however, a more or less pronounced loss of toughness is observable, restricting the range of applications, especially in interrupted cutting. This advantage can be overcome by using the physical vapour deposition (PVD) process. This paper uses the example of arc-deposited Ti-Zr-N coatings on cemented carbide indexable tips to illustrate the superior performance of PVD-coated as opposed to CVD-coated and uncoated cemented carbides in certain applications. The coating-substrate properties are related to coating parameters and film composition and correlated with tool lives in interrupted cutting. Coating parameters must evidently be selected to achieve a low process temperature and the most saturated stoichiometry possible. Ternary Ti-Zr-N coatings have some advantages over binary Ti-N or Zr-N films.

Systematic approach to improve the performance of PVD coatings for tool applications

Abstract For some years indexable tips coated by physical vapour deposition (PVD) have been tested regarding their performance characteristics. Beyond this there is great interest in PVD coatings for new applications in the field of tool technology. The variety of application conditions (principles) and the multitude of parameters complicate the establishement of PVD technologies in these fields. This paper describes a systematical approach which is useful for new developments of PVD-coated tools and has already been proven successful for various applications. Furthermore, a number of examples are presented to which this systematic approach has been applied. Besides applications in cut-machining operations, e.g. milling, drilling, turning and rasping, there are also tools applied within the areas of production die casting, forging, feeding or measuring, mostly working under extreme conditions (high temperature corrosion, thermal shock, adhesive wear and abrasion).

Diffusion barrier coatings with active bonding, designed for gas turbine blades

Abstract A physical vapour deposition diffusion barrier coating, based on amorphous alumina, offers the possibility to reduce the interdiffusion between the MCrAlY overlay and the Ni-base material occurring at elevated temperatures. The barrier function depends on the high temperature stabilization of the dense, amorphous structure. The coating contains nitrogen against the crystallization of alumina phases. Owing to the fact that todays MCrAlYs need diffusion bonding for a satisfactory adhesion, the barrier is deposited as X-Al-O-N, with X as a reactive element forming a diffusion bond layer with the Ni-base and MCrAlY during the first thermal treatment. The paper reviews several high temperature tests in which the barrier function of the alumina based coating was proved. The bonding effect of the chosen X element is explained. The influence of a deposition parameter variation on the formation of the reactive bonding zone is investigated. With this diffusion barrier, the temperature level of the first stage turbine blade could be increased up to 1100°C in the MCrAlY-Ni-base interface.

Multicomponent and multilayer physically vapour deposited coatings for cutting tools

Abstract The physical vapour deposition (PVD) processes are characterized by high flexibility in terms of the film systems which can be deposited, The hard phases of the multicomponent and multilayer films can be formed from any desired metal and metalloid components. PVD films can also be deposited onto virtually any substrate material or workpiece geometry. The paper reviews PVD processes suitable for tool coating applications. It then characterizes advantages of multicomponent and multilayer coatings and describes the coating-tool combinations which are most widely used in industry and for which research results indicate the greatest future application potential. Particularly promising in this respect are PVD coatings which achieve significantly prolonged tool lives in interrupted cut machining, such as Ti−Zr−N, Ti−C−N and TiN/Ti−C−N multilayer coatings.

Deposition of arc TiAlN coatings with pulsed bias

Abstract It is a well-known fact that the aluminium content of TiAlN coatings deposited with the arc physical vapour deposition (PVD) process depends mainly on substrate potential and source-to-substrate distance. To achieve good results in cutting operations with TiAlN-coated tools with a low aluminium content in the film, it is necessary to deposit TiAlN with a relatively high bias voltage which raises the substrate temperature to a level which can cause some damage to the structure of even high speed steel substrates. For high performance cutting operations with TiAlN thin films a high and homogeneous aluminium content in the films, especially on cutting edges, is necessary. Higher aluminium content in arc PVD thin films is achieved with lower bias voltage during deposition which in turn lowers the deposition temperature and consequently enables heat-sensitive substrates to be coated; however, for good adhesion of the deposited films a high bias voltage is required. The application of a pulse bias generator instead of a d.c. bias offers the possibility to decrease the deposition temperature and to obtain more aluminium in the coating. In this paper we compare coating properties of TiAlN deposited with d.c. and pulsed bias source. With pulsed bias it is possible to achieve higher aluminium content in the coating, especially on cutting edges. Analysis of increase in aluminium on cutting edges was carried out by energy-dispersive X-ray analysis.