G. Krämer

On the origin of compressive stress in PVD coatings — an explicative model

Abstract Compressive stress in hard plasma vapor deposition coatings — up to values of several gigapascals — is found to be strongly dependent on the process parameters, especially on the energy of ion bombardment on the growing film. Increasing energy usually causes increasing stress. Internal stress influences all other coating properties, e.g. adhesive strength, microhardness and wear resistance. A detailed knowledge of the origin of this stress is therefore of great significance, particularly for practical applications. Experimental work was performed with various coating and substrate materials and various coating processes (d.c. sputtering, r.f. sputtering, arc ion plating) to determine the interrelationships. The creation of internal stress can be explained by a phenomenological model describing the interaction of bombarding ions in the plasma and coating ions, whose mobility is restricted by their position on the substrate or previously deposited layers of the coating. The validity of the model is demonstrated by good agreement with experimental results for both crystalline and amorphous coatings.

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.

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.

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.

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.

Arc deposition of TiC and TiCN using acetylene as a reactive gas

Abstract Thin hard coatings on cutting tools and metal-forming dies now contribute substantially to the reduction of tool wear. They permit cost-effective machining processes and open the way to new tool applications. Titanium carbonitride physical vapour deposition (PVD) coatings have demonstrated their high performance potential in interrupted cut machining. Hitherto, however, production applications have been limited, due to technical difficulties. On the basis of detailed research into the stability of the PVD arc evaporation process, the paper demonstrates that TiC and TiCN coatings can be deposited successfully and reproducibly on cemented carbides, using acetylene as the carbon-containing gas. The structures and properties of the coatings are described as a function of dominant process parameters. It can be shown that substrates temperatures of approximately 500°C are required for titanium to react with carbon. Arc-PVD coatings produced with acetylene consequently attain high hardness values. Due to high internal compressive stress, however, pure TiC coatings tend to spall. Conversely, very little internal stress occurs in TiCN coatings, resulting in good coating-substrate adhesion.

Abrasive wear resistance and cutting performance of complex PVD coatings

Abstract In recent years the development of physical vapour deposition coatings for improved wear resistance of cutting tools is turning to more complex coating systems. Different multilayer systems are offered in addition to binary, ternary and quaternary monolayer coatings. Their variety makes it impossible to test all possible coatings under real machining conditions. Model wear tests such as the Taber Abraser test can help in preselecting the coatings with a good potential to perform successfully. In this paper we compare the results of the Taber Abraser test of different multicomponent and multilayer coating combinations within the system Ti-Zr-C-N with the results of cemented carbide indexable tips in interrupted-cut machining. It is shown that the results of both examinations often correlate and that abrasive wear tests can be used to give information about the potential of coatings under special cutting conditions.

Substrate- and interface-related influences on the performance of arc-physical-vapour-deposition-coated cemented carbides in interrupted-cut machining

Abstract In recent years, coated cemented carbides have become increasingly important in the machining of hard steel materials. Modern physical vapour deposition (PVD) processes in particular allow the production of tools combining high wear resistance with great toughness. Owing to the complex load spectrum involved in the machining process, performance is influenced by a wide range of factors. Together with the choice of coating composition and process parameters, substrate pre-treatment and substrate properties are significant. The paper uses the example of arc-PVD-coated cemented carbide indexable tips to investigate the effects on the composite properties and on the tool performance in the interrupted-cut machining of the substrate material, substrate surface quality and ionic etching used to initiate the coating process. It is evident that the cutting-speed-dependent tool life characteristic of the coated indexable tips is influenced to differing degrees by the various factors. A higher substrate toughness, the creation of compressive stress in substrate surface zones and substrate-protective ionic etching reduce the risk of cutting-edge chipping.

PVD coatings on aluminium substrates

Abstract It is well known that aluminium is today the most widely used metallic material besides steel. The mechanical characteristics of aluminium offer an increasing application field, especially where lightweight constructions are required. The demands for improved characteristics such as higher strength and greater durability are achieved by the development of new aluminium alloys. Continuous efforts are made in research into new possibilities for making use of the advantages of aluminium in applications that were reserved up to now for harder and more wear-resistant materials. Because of their environmental benefits modern PVD processes represent a better alternative to a number of conventional coating processes to deposit wear-resistant films on aluminium surfaces. The aim of this paper is to describe the possibilities of TiN coating on an aluminium alloy, AlMgSi1, by application of the arc-PVD process without damage to the heat-sensitive substrate material. Different machining and cleaning processes were used for the preparation of the aluminium substrates. The relationship between substrate pretreatment and coating characteristics is demonstrated. The influence of different process parameters such as etching, bias voltage and coating time as well as the arrangement of the substrates in the vacuum chamber were investigated. Multilayer (Ti and TiN) and graded coatings were deposited on the aluminium alloy to achieve the best results in deposition rate, microhardness and critical scratch load. To characterize the abrasive wear behaviour of the coated aluminium substrates a special sandblasting test was used. These results are also described in this paper. Finally a suitable process technology is provided for the application of thin protective films on aluminium in order to increase wear resistance.

Performance behaviour of physical-vapour-deposition-coated cermets in interrupted-cut machining

Abstract This paper describes the performance behaviour of arc-physical-vapour-deposition-coated cermets in interrupted-cut machining as a function of the coating-substrate combination and the coating parameters. It also compares performance characteristics with those of coated carbides. One important result emerging from the study is that coated cermets achieve higher cutting speeds than those for coated carbides in interrupted cutting. It is also apparent that the toughness of the substrate grades, the tribological properties of the coating material and the process temperature of the coating process are all highly significant for the service properties of the cermet tools in interrupted-cut machining. Given suitable substrate qualities, tool lives can be increased significantly by physical vapour deposition coatings, especially by Ti-Zr-N coatings.