E.D. Doyle

A study of the wear mechanisms of Ti1−xAlxN and Ti1−x−yAlxCryN coated high-speed steel twist drills under dry machining conditions

Abstract Recent advances in cutting tool materials and PVD surface coatings are making high-speed dry machining an increasingly viable machining operation in commercial manufacturing sectors. High oxidation resistant coatings, such as TiAlN, are used extensively in global manufacturing for reducing production costs and improving productivity in such aggressive metal-cutting operations. In this investigation, the performance of TiN, Ti1−xAlxN and Ti1−x−yAlxCryN coatings was assessed on Co-HSS twist drills used to machine an automotive grade grey cast iron. As part of the study, two experimental methods were used to characterise the performance of the twist drills, namely, by an audible ‘screech’ and measurement of the progression of outer corner flank wear lands. Both techniques showed that a single layer TiN coating failed to outperform even uncoated twist drills when dry machining grey cast iron under aggressive machining conditions. However, using the measurement of outer corner flank wear technique, the average tool life of uncoated twist drills was increased by factors of 4.5, 6.5 and 6.9 by Ti0.59Al0.41N, Ti0.27Al0.19Cr0.54N and Ti0.21Al0.14Cr0.65N coatings, respectively. In the case of ‘screech’ failure, the average tool life of uncoated twist drills was increased by factors of 3.9, 7.3 and 10.8 by Ti0.59Al0.41N, Ti0.27Al0.19Cr0.54N and Ti0.21Al0.14Cr0.65N coatings, respectively. The performance of twist drills was also assessed using a number of quantitative techniques, namely, measurement of thrust and torque, hole diameter and drill wear. The latter was also observed using scanning electron microscopy (SEM) which showed an early onset of workpiece material transfer at the margins of the uncoated drills. In the case of the PVD coated drills, an improvement in the tribological interaction between the coatings and the grey cast iron workpiece resulted in a significant reduction in material transfer at the drill margins. The above results are discussed in terms of their affect on hole diameter and cutting forces.

Influence of chromium content on the dry machining performance of cathodic arc evaporated TiAlN coatings

Physical vapour deposition (PVD) titanium aluminium nitride coated cutting tools are used extensively in global manufacturing for reducing production costs and improving productivity in a number of aggressive metal-cutting operations, namely, dry and high-speed machining. In this investigation, the performance of Ti1−xAlxN and Ti1−x−yAlxCryN coatings was assessed on Co-HSS twist drills used to machine grey cast iron. The failure criterion for drills was defined as a critical sized flank wear land at the outer corners of the drills. Using this criterion, the average tool life of uncoated twist drills was increased by factors of 2.5, 3.0 and 3.0 by Ti0.59Al0.41N, Ti0.27Al0.19Cr0.54N and Ti0.21Al0.14Cr0.65N coatings, respectively. Notwithstanding the similar increase in average tool life, the Ti1−x−yAlxCryN coatings produced more consistent results than the Ti1−xAlxN coated drills with standard deviations of 67, 3 and 19 holes, respectively. This result has significant practical implications in manufacturing, since drills are not replaced on an individual basis, but rather on a preset tool change frequency. The present paper discusses the performance of Ti1−xAlxN and Ti1−x−yAlxCryN coated drills in terms of average and practical drill life and concludes with remarks on the characterisation of PVD coatings and their significance on the performance of Co-HSS twist drills when dry machining grey cast iron.

Characterisation and performance of partially filtered arc TiAlN coatings

It is now being recognised that the level of residual stress in physical vapour deposited (PVD) coatings is yet another important physical property of thin film PVD coating parameters that can critically affect their performance. In this study, titanium aluminium nitride (TiAlN) coatings were deposited using a dual source filtered arc evaporation system with a view to determine the effect of bias voltage increased from −50 to −250 V, the residual stress in the coating as well as the adhesion and cutting tool performance. The X-ray diffraction methods of Bragg–Brentano and glancing angle parallel beam (sin2ψ) were used to study the texture and residual stress in the as-deposited coatings. The results showed that as the bias voltage increased from –50 to –250 V and the residual stress increased from 7.67 to 11.81 GPa (compressive). However, little change in residual stress was observed with an increase in arc current from 75 to 175 A. All coatings exhibited a preferred orientation in the {111} direction, however, a reduction in the {111} intensity was observed for the coating deposited at –50 V. Coating hardness was observed to increase from 26.3 to 31.7 GPa when the bias voltage was increased from –50 to –150 V. However, no further increase was observed at –250 V. No effect on hardness was noted for any change in arc current. Scratch adhesion results showed little effect on the bias voltage with critical load values of 46, 48 and 43 N for the −50, −150 and −250 V biased coatings, respectively. However, increasing the arc currents above 75 A resulted in a reduction of critical load from 48 N for 75 A to 36 and 37 N for 125 and 175 A arc currents, respectively. Damiler Benz Rockwell ‘C’ adhesion tests carried out also revealed a similar trend. The Al content of the coating was found to decrease with increasing bias voltage but increase with an increase in arc current. Accelerated drill life tests suggested that an increase in residual stress associated with the higher bias voltages does affect tool life. For arc current variation, the balance between intrinsic and extrinsic stresses that contribute to the total residual stress may affect cutting tool life.

Dry cutting performance of partially filtered arc deposited titanium aluminium nitride coatings with various metal nitride base coatings

Abstract PVD titanium aluminium nitride coatings are known to improve the performance of cutting tools in aggressive machining applications, such as high-speed dry machining. This is generally acknowledged to be due to the fact that these coatings are able to maintain high hardness and resistance to oxidation at the elevated temperatures typically experienced in such metal cutting applications. In this investigation, the performance of single layer coatings of TiN and CrN, and double layer coatings of TiN/TiAlN and CrN/TiAlN were assessed on Co-HSS drills used to machine grey cast iron. It was found that the CrN coatings performed as well as the TiN coatings when drilling grey cast iron. In addition, the double layer coatings outperformed the single layer coatings with the CrN/TiAlN coatings having a 20% improvement in tool life compared with the TiN/TiAlN-coated drills. This result is particularly interesting since the Daimler–Benz indentation test suggested that the TiN/TiAlN had much lower adhesion than the CrN/TiAlN. These results are discussed in terms of an analysis of the physical and mechanical properties of the coatings but it highlights the need for meaningful measurement of critical coating properties which might relate to their performance in metal cutting.

Performance evaluation of PVD coatings for high pressure die casting

Abstract During high-pressure die-casting (HPDC) of aluminium alloys, there is a tendency for the molten alloy to react with the tool steel die, core pins and inserts. This occurrence within the high pressure die casting (HPDC) industry is referred to as ‘soldering’. It is of concern to high-pressure die casters because of down-time due to the regular removal of the soldered layer and its detrimental affect on die life and casting quality. In this investigation, several physical vapour deposited (PVD) coatings, namely, TiN, CrN and TiCN, were evaluated for their ability to eliminate soldering during HPDC of aluminium alloys. Accelerated semi-industrial trials were carried out in a 250-t Toshiba HPDC machine using a specially designed die made of P20 tool steel with removable core pins. The results from these trials showed that PVD coatings can act as a physical barrier coating preventing any reaction between the molten aluminium alloy and the tool steel. Thus the problem of soldering on such tools as core pins can be eliminated in high HPDC of aluminium alloys. In the accelerated trials, it was found that soldering was replaced by a built-up layer of cast aluminium alloy, which was less detrimental to tool life and reduced machine down-time due to the reduced need for tool polishing. The experimental results were confirmed by conducting in-plant HPDC trials.

Dry machining : commercial viability through filtered arc vapour deposited coatings

Abstract Critical to the economic viability of dry machining is justifying increased tool costs in terms of productivity and ecological savings achievable through the elimination of metal cutting fluids. The automotive industry is a large-scale commercial manufacturing sector in which emphasis is placed on reducing impact on the environment as well as seeking to reduce costs in manufacturing. This paper presents results for dry drilling fully pearlitic grey cast iron with uncoated and partially filtered arc deposited TiN and TiAlN coated Co-HSS split point twist drills using two methods to distinguish drill failure, namely audible screech and measurement of outer corner wear lands. The latter method proved to be of most relevance in manufacturing because it is more closely related to the industrial practice of on-condition assessment prior to drill resharpening. Using the outer corner wear method, TiN coatings (3.0 μm thickness) achieved a modest increase in drill life of 1.4 times, compared with significant improvements of 2.6 times, achieved from a TiAlN top coating (0.8 μm thickness) on a base coating of TiN (1.2 μm thickness). This is a novel result in that a thin top coating of the expensive TiAlN coating over the much cheaper TiN coating can provide a very cost-effective tool coating combination.

Characterisation of TiN and TiAlN thin films deposited on ground surfaces using focused ion beam milling

TiN and TiAlN/TiN PVD coatings deposited onto as-ground surfaces have been characterised via direct cross-sectional imaging and transmission electron microscopy with the aid of a focused ion beam system. Cross-sections showed that the coatings exhibit consistent coverage, even in sheltered areas such as at the base of grooves resulting from prior grinding. A columnar grain structure was observed in all coatings. A number of defects were observed such as seams and voids resulting from coating onto the as-ground surface and macroparticles which were shown to be deposited during the metal ion etching stage. Cross-sections through nanoindents revealed that the coatings deform by through-thickness shear cracking.The combination of the excellent coverage provided by PVD and the deformation mechanisms, which are related to the microstructure, contribute to the excellent performance of these coatings that has lead to their widespread application.

Intrinsic Stress of DLC Film Prepared by RF Plasma CVD and Filteredcathodic ARC PVD

Summary form only given. Diamond-like carbon (DLC) films frequently exhibit poor adhesion strength and delaminate instantly due to high internal compressive stress , as high as 8.5 GPa generated in the film, thus resulting in the limitation of the film. Our recent experimental results suggest a functional relationship between the intrinsic compressive stress and the negative biasing voltage applied to the substrate. For the first time, we have obtained DLC compressive stress data as a function of DC bias voltage for films prepared from the C2H2 RF plasma and we compare them with data obtained from the cathodic arc. Although the deposition rate was different, the DLC deposition rate of the filtered cathodic arc being ~1 nm per sec. while the rate in the RF process is approximately 2 nm per min., a similar trend in the stress generation and the stress-relief region was observed in both methods of deposition. The motivation for this study was to establish a coating methodology for DLC that yields a high sp3:sp2 ratio and strong adhesion strength. Such a coating is expected to be hard but not easily delaminated, and would be useful when coated on to steel substrates such as industrial cutting tools, to enhance life performance and cost savings. The preliminary results showed that a thicker DLC film can be obtained by incorporating a lower stress, graphitic layer or a silicon layer.