M.A. Djouadi

CrxNy hard coatings deposited with PVD method on tools for wood machining

Abstract Although well known and widely used, tools modified with hard coatings are still not applied to the wood industry. Difficulties come from mechanical, physical and tribological properties of wood and of wood based materials together with particular parameters of their processing. The aim of this study was to synthesise hexagonal and cubic chromium nitride coatings and apply them in wood processing to improve cutting tools’ wear resistance. The apparatus used was an industrial type router. The material subjected to cutting was OSB (Oriented Strand Board). The cutting tools with an edge angle of 55° were cemented carbide inserts. The coatings were deposited by triode sputtering system; the hexagonal phase was obtained for 10% of nitrogen in the Ar+N 2 mixture used in the discharge, the cubic one was obtained for a percentage higher than 20%. The composition of coatings was determined by EDS and WDS microanalysis and SEM cross-section observations were performed. The mechanical properties of layers were determined by stress measurements and by a wear test performed with in situ method. It reveals that the most resistant coatings are the Cr 2 N films. These optimal conditions of deposition have been applied to cutting tools in routing process. It appears that CrN or Cr 2 N coatings deposited on both tools’ faces ameliorate significantly tools wear resistance.

Stress profiles and thermal stability of CrxNy films deposited by magnetron sputtering

Abstract CrN and Cr2N coatings were synthesized by magnetron sputtering. The limiting factors for the application of these coatings for protecting cutting tools are their lack of adhesion and thermal stability. Therefore, a study of these two parameters has been conducted. First, the influence of the film thickness on the compressive stress and also on the mechanical and structural properties of the films were studied. The results showed that for both phases the stress is not homogeneous with the film thickness. Second, the thermal stability was investigated by post deposition annealing in nitrogen. It reveals that CrN and Cr2N films are stable when heated at temperatures up to 1000 K.

Wear resistance of coatings in high speed gear hobbing

Abstract Coating technology is one means of achieving a crucial enhancement in tool performance, especially in hobs that were among the first tools to be coated on a large scale. Nevertheless only few detailed analysis of wear mechanism have been done on field machines. The bifunctional coatings (combination of a tough, hard and refractory coating and of a self lubricating coating possessing a good thermochemical and abrasion resistance but a lower hardness) are very interesting since it is difficult to get a simple coating showing all these characteristics. The use of bilayer coatings raises several problems especially for dry and high speed cutting. Therefore, in order to investigate the behaviour of these bifunctional coatings, hobs have been coated by physical vapour deposition (PVD) methods. After the elaboration of a procedure for hobs testing, field tests have been performed. Results of tool life tests and investigations on tool wear mechanisms for different coated hobs are presented and discussed. The interesting performance in high speed gear hobbing of sintered high speed steel (HSS) hobs (ASP2052) combined with a (Ti,Al)N+MoS2 coating is particularly underlined.

Influence of CrxNy coatings deposited by magnetron sputtering on tool service life in wood processing

Abstract Cubic CrN(200) and a mixture of 25% CrN+75% Cr 2 N(111) coatings were deposited by magnetron sputtering onto carbide tools and tested in wood machining. The composition of the layers was determined by X-ray photoelectron spectroscopy (XPS) analyses. Scanning electron microscopy (SEM) observations showed that Cr 2 N layers have a columnar structure while CrN layers seem to be featureless. Secondary ion mass spectroscopy (SIMS) profiles have shown a good homogeneity of the layers and no interdiffusion of the different species into the substrate. The mechanical properties of the chromium nitride coatings were characterized by residual stress measurement, hardness measurements and scratch adhesion tests. The layers have some compressive stresses in the range −1.7/−2.1 GPa and −0.4/−1 GPa for CrN and Cr 2 N, respectively. The more adherent layers are CrN ones which exhibit hardness up to 1900–2100 HV. Cr 2 N layers are harder (up to 1900–2100 HV) but less adherent. Machining tests were performed up to 5 and 10 km of cutting distance with industrial routers on Oriented Strand Board. Tool wear and service life were compared between the uncoated and coated tools. First, it has been observed that the edge wear is very high, just at the beginning of the routing process. The best result was shown by a carbide tool CrN coated on both faces; the service life of this tool is four times higher than the one of an uncoated tool.

The influence of deposition parameters on the wear resistance of CrxNy magnetron sputtering coatings in routing of oriented strand board

Abstract Cubic CrN coatings were deposited by magnetron sputtering onto carbide tools and tested in wood machining operations. The coatings composition was determined by Energy Dispersive Spectroscopy. SEM cross-section observations verified their morphology. The phase structure of the layers was determined by XRD analyses. In the first step, the mechanical properties of the chromium nitride coatings were characterised by hardness measurements and scratch adhesion tests. In the second step, carbide tools were tested in machining of Oriented Strand Board (OSB) up to 10 km of cutting distance using an industrial router. The tool wear was quantified by recording and analysing the cutting edge profiles at increasing cutting distances. These observations were performed using the Vertical Scanning Interferometry technique on 3D surface profiler (Wyko NT 2000). Parameters such as, the bevel width at 0.1 mm depth from the tip and the nose loss were computed for each profile. The influence of deposition conditions, such as the coatings thickness, the working pressure and the target power, on the mechanical and structural properties of coatings and the wear of the coated carbide tools was studied. It has been found that CrN coatings, obtained with a thickness of 2 μm at working pressure of 0.2 Pa, and a target power of 650 W performed the best in OSB machining. A good agreement was observed between the tribological properties and the wear rate in wood machining tests. An influence of the layer structure was also observed: CrN crystallised with (111) and (200) orientations, performed better in OSB machining than the one presenting (200)-preferred orientation.

Modification by composite coatings of knives made of low alloy steel for wood machining purposes

Abstract Low alloy steel (LAS) is a commonly used material for producing knives in wood and pulp industry. In some application specific advantageous features of LAS make it impossible to replace it by other harder and more resistant to abrasion materials such as: HSS, carbide or stellites. The aim of this study was to improve the resistance to wear of knives’ edges made of LAS covered with composite coatings (duplex coatings). This method is based on low temperature ion nitriding, when effects of the modification are intensified with deposition of hard films on pre-nitrided knives’ surfaces. The results of the research show that composite modification with Cr X N film deposited on pre-nitrided surfaces enables improving the knives’ resistance to wear. The Cr X N hard film gives an added improvement effect to the wear resistance of nitrided knives. The tools service life in fast wear tests rises more than 2.5 times. The modification of knives’ edges also improves their resistance to shocks caused by the dynamic forces active during the cutting process.

Duplex treatment of 32CrMoV13 steel by ionic nitriding and triode sputtering : Application to wood machining

Abstract The efficiency of hard coatings, such as Cr x N y , in increasing abrasion resistance of cutting tools for peeling has been shown in previous studies [Surf. Coat. Technol. 74–75 (1995) 625–633; Meca. Indus. 3 (2002) 333–342]. Furthermore, it has also been demonstrated that this efficiency was increased by a previous thermal treatment of the steel cutting knife that means by a duplex treatment consisting in nitriding followed by CrN-sputtered coatings [Meca. Indus. 3 (2002) 333–342]. The aim of the study is to optimise the application of duplex treatments by modifying the nitriding gas mixture in high temperature process. The influence of gas mixture (composed of N 2 , H 2 and CH 4 ) on the mechanical and structural properties of 32CrMoV13 low alloy steel samples was studied. The composition and structure of the nitriding layer was determined by the EDS and the XRD, respectively. Vickers microhardness profiles were also performed to study the influence of the gas mixture. The morphology of the nitriding layers was observed by optical microscopy. Cubic Cr x N y coatings have been realised by triode sputtering on both faces of the cutting knives. The EDS analyses permitted to verify the composition of the layers while their structure was determined by the XRD. Different kind of cutting knives were tested in peeling of non-defect beech wood: non-treated, nitrided, CrN-triode sputtered and CrN-duplex treated cutting knives. The friction coefficient between the cutting material and the wood was determined and the wear of the knive edge (reduction of the edge) was also measured. It was obvious that increasing the nitrogen contents from 20 to 80% in the nitriding gas mixture N 2 + H 2 or adding 5% of methane permit to increase the nitrided layer’s thickness and hardness. The nitrided cutting knives and CrN-triode sputtered duplex based treated knives were tested and compared in peeling of beech. The duplex treated knives obtained with 80% of N 2 or 5% of CH 4 performed best and allowed to increase almost twice the tools service life.