Properties,applications and manufacture of wear-resistant hard material coatings for tools

Wear processes during machining with cemented carbides and the effect of coatings on wear and on toughness are described. The requirements for coatings relating to optimum tool life are reviewed. Recently developed multilayer coatings with and without ceramic layers are described and examples for improvements in the tool lifetime are given. Additionally, some examples for the application of coatings for milling and chipless forming are presented.The wear-reducing effect of coatings on steel tools is described and possibilities for applications of steel tools coated using chemical vapour deposition (CVD) are elucidated. A number of examples for coated cutting and chipless forming steel tools are described.Finally, useful processes for the coating production are considered. A comparison between physical vapour deposition and CVD processes with respect to the coating temperature, the consistency and the adhesion of the coating is presented. The CVD process for economic production is considered and a suitable coating unit is outlined.     

Performance evaluation of reactive direct current unbalanced magnetron sputter deposited nanostructured TiN coated high-speed steel drill bits

The stainless steels, in general, are considered to be difficult-to-machine materials. In order to machine these materials the surface of the tool is generally coated with physical vapour deposition (PVD) hard coatings such as titanium nitride (TiN), titanium aluminum nitride (TiAlN), etc. The adhesion is of vital importance for the performance of tools coated with PVD coatings. Proper surface treatments (in situ and ex situ) are required to achieve highly adherent PVD coatings on tools. We have deposited nanostructured TiN coatings on high-speed steel (HSS) drill bits and mild steel substrates using an indigenously built semi-industrial four-cathode reactive direct current (d.c.) unbalanced magnetron sputtering system. Various treatments have been given to the substrates for improved adhesion of the TiN coatings. The process parameters have been optimized to achieve highly adherent thick good quality TiN coatings. These coatings have been characterized using X-ray diffraction, nanoindentation and atomic force microscopy techniques. The performance of the coated HSS drill bits is evaluated by drilling a 13 mm thick 304 stainless steel plate under wet conditions. The results show significant improvement in the performance of the TiN coated HSS drill bits.     

金刚石涂层与硬质合金刀具附着力的研究进展

介绍了国内外在低压合成金刚石涂层刀具中提高薄膜基体附着力的几种典型工艺方法。给出了ＷＣ－Ｃｏ基底刀具预处理对低压合成金刚石薄膜附着力的影响。     

Effects of Ti- and Zr-based interlayer coatings on the hot filament chemical vapour deposition of diamond on high speed steel

The prospect of obtaining good adhesion of diamond films onto steel substrates is highly exciting because the achievement of this objective will open up applications in the cutting and drilling industry. However, a major problem with depositing diamond onto steel is high diffusion of carbon into steel at chemical vapour deposition (CVD) temperatures leading to very low nucleation density and cementite (Fe₃C) formation. Therefore, the study of the nucleation and growth processes is timely and will yield data that can be utilised to get a better understanding of how adhesion can be improved. This work focuses on investigating the adhesion of thin diamond films on high speed steel previously coated with various interlayers such as ZrN, ZrC, TiC and TiC/Ti(C,N)/TiN. The role of seeding on nucleation density and the effect of diamond film thickness on stress development and adhesion has been investigated using SEM, XRD and Raman spectroscopy.The main emphasis in this study is the TiC interlayer which for the first time proved to be a suitable layer for diamond CVD on high speed steel (HSS). In contrast from other interlayer materials investigated here, no delamination was observed even after 3 h of CVD at 650 °C only when TiC was employed. Nevertheless, the increase of diamond film thickness on TiC coated HSS substrates led to the delamination of small areas in various regions of the substrate. This occurrence suggests that there was a distribution of adhesive toughness values at the diamond/TiC interface with stress development being dependent on film thickness.     

Fluidized bed micro-machining and HFCVD of diamond films onto Co-cemented tungsten carbide (WC-Co) hardmetal slabs

The effect of fluidized bed (FB) treatment upon hot filament chemical vapor deposition (HFCVD) of polycrystalline diamond films onto WC-Co hardmetal substrates was investigated. Several scenarios to make the substrates ready for HFCVD were, comparatively, evaluated and the resulting diamond films were examined in terms of their morphology and adhesion. The diamond grain density was measured by scanning electron microscopy. The adhesion of continuous diamond film to substrate was evaluated by the reciprocal of the slope of crack radius-indentation load functions. Surface binder dissolution followed by FB treatment (PF pretreatment) allowed very high diamond nucleation density and smaller grain size. The adhesion of films grown on PF pretreated substrates was found to be very close to that of films deposited on hardmetal slabs pretreated by Murakami's reagent followed by Co etching with Caro's acid and seeded with diamond suspension in an ultrasonic vessel (MPS pretreatment). However, diamond coatings on MPS pretreated samples exhibited a rougher surface morphology as a result of both lower diamond nucleation density and larger substrate surface roughening by Murakami's etching. Based upon experimental findings, our newly developed PF pretreatment was found to be a very promising technique in substrates conditioning as well as in promoting adherent, uniform and smooth diamond coatings onto hardmetal tools and wear parts.     

Chemical vapour deposition of microdrill cutting edges for micro- and nanotechnology applications

Conventional cemented tungsten carbide-cobalt (WC-Co) microdrills generally have a low cutting efficiency and short lifetime mainly because they operate at very high cutting speeds. Since it is relatively expensive to make microtools it is highly desirable to improve their lifetime and in-service performance. Microtools used to make microelectronic and mechanical systems (M.E.M.S) devices with sharp cutting edges, such as milling or drilling tools need protective coating in order to extend life and improve performance. One method of achieving this objective is to use a suitable surface engineering technology to deposit a hard wear resistant coating, such as diamond. Diamond has excellent mechanical properties, such as ultra-high hardness and a low friction coefficient. One of the most promising surface treatment technologies for depositing diamond onto complex shaped components is chemical vapour deposition (CVD). However, CVD of diamond coatings onto the cemented WC-Co tool has proved to be problematic. Binder materials such as cobalt can suppress diamond nucleation resulting in poor adhesion between the coating and substrate. In this paper the effects of pre-treated substrate material on the coating structure are reported. The morphology and the crystallinity of the as-grown films was characterised by using scanning electron microscopy (SEM). Raman spectroscopy was used to assess the carbon-phase purity and give an indication of the stress levels in the as-grown polycrystalline diamond films. The diamond coated tools have potential applications in micro- and nanomachining of micro- and nano-sized components used in M.E.MS.     

Plasma-assisted deposition of metal and metal oxide coatings

A combination of physical vapour deposition and plasma-assisted chemical vapour deposition techniques were used to deposit Cu-, Ni- and Sn-rich SnO/SnO2 coatings on metal and ceramic substrates. Cu and Ni were deposited on Al alloy 6061 substrates and Ni deposition was also performed on glass microscope slides and commercially pure alumina substrates. Sn-rich SnO/SnO2, on the other hand, was coated on stainless steel and pure Cu substrates. A direct-current plasma system was used to deposit the pure metals in vacuum with a resistively heated tungsten boat that was coated with alumina. All samples were sputtered for 20 min in an argon:hydrogen (1:1) atmosphere at a pressure of 300 mTorr. To reduce contamination and oxidation of both substrates and deposited layers, Cu and Ni coatings were made with argon:hydrogen (2:1) carrier gas. Sn-rich tin oxide coatings were deposited in a pure argon atmosphere (no hydrogen) to allow for the oxidation of Sn deposits on the stainless steel and copper substrates. Investigations of coated surfaces by scanning electron microscopy and X-ray diffraction showed coatings to be smooth, continuous and pure. Deposition rates showed this application to provide a very high rate when compared with chemical vapour deposition and metal–organic chemical vapour deposition techniques. Scratch tests results prove good attachment of the coatings to their respective substrates. © 1998 Chapman & Hall     

Effect of methane concentration on physical properties of diamond-coated cemented carbide tool inserts obtained by hot-filament chemical vapour deposition

Diamond-coated tools can greatly improve the productivity of machining highly abrasive materials such as high silicon–aluminium alloys used in the automotive industry. Cemented-carbide diamond-coated tool inserts have not become an off-the-shelf product owing to several difficulties including insufficient adhesion of diamond to the substrate and questionable reproducibilty in their machining performance in the manufacturing. In order to overcome these difficulties, a better understanding of the effects of the chemical vapour deposition (CVD) conditions such as methane concentration, reactor pressure and substrate temperature is important. In this work, cemented tungsten carbide tool inserts with 6 wt% Co (WC–Co) were coated with diamond films deposited at five different methane concentrations (1–9 vol%). Here we present preliminary results of the effect of methane concentration variation on the following physical properties of the diamond coating: surface morphology; crystal structure; chemical quality; surface roughness; residual stress. The results indicate that the best physical properties of diamond-coated tool inserts using hot-filament CVD are achieved with diamond coatings deposited at methane concentrations ranging from 1 to 3%.     

The versatility of hot-filament activated chemical vapor deposition

In the field of activated chemical vapor deposition (CVD) of polycrystalline diamond films, hot-filament activation (HF-CVD) is widely used for applications where large deposition areas are needed or three-dimensional substrates have to be coated. We have developed processes for the deposition of conductive, boron-doped diamond films as well as for tribological crystalline diamond coatings on deposition areas up to 50 cm × 100 cm. Such multi-filament processes are used to produce diamond electrodes for advanced electrochemical processes or large batches of diamond-coated tools and parts, respectively. These processes demonstrate the high degree of uniformity and reproducibility of hot-filament CVD. The usability of hot-filament CVD for diamond deposition on three-dimensional substrates is well known for CVD diamond shaft tools. We also develop interior diamond coatings for drawing dies, nozzles, and thread guides.Hot-filament CVD also enables the deposition of diamond film modifications with tailored properties. In order to adjust the surface topography to specific applications, we apply processes for smooth, fine-grained or textured diamond films for cutting tools and tribological applications. Rough diamond is employed for grinding applications. Multilayers of fine-grained and coarse-grained diamond have been developed, showing increased shock resistance due to reduced crack propagation.Hot-filament CVD is also used for in situ deposition of carbide coatings and diamond-carbide composites, and the deposition of non-diamond, silicon-based films. These coatings are suitable as diffusion barriers and are also applied for adhesion and stress engineering and for semiconductor applications, respectively.     

Diamond coated dental bur machining of natural and synthetic dental materials

Diamond coatings are attractive for cutting processes due to their high hardness, low friction coefficient, excellent wear resistance and chemical inertness. The application of diamond coatings on cemented tungsten carbide (WC-Co) burs has been the subject of much attention in recent years in order to improve cutting performance and tool life. WC-Co burs containing 6% Co and 94% WC with an average grain size 1–3 micron were used in this study. In order to improve the adhesion between diamond and the bur it is necessary to etch away the surface Co to prepare it for subsequent diamond growth. Hot filament chemical vapour deposition (H.F.C.V.D.) with a modified vertical filament arrangement has been employed for the deposition of diamond films. Diamond film quality and purity has been characterised using scanning electron microscopy (S.E.M.) and micro-Raman spectroscopy. The performance of diamond coated WC-Co burs, uncoated WC-Co burs, and diamond embedded (sintered) burs have been compared by drilling a series of holes into various materials such as human teeth, and model tooth materials such as borosilicate glass and acrylic. Flank wear has been used to assess the wear rates of the burs when machining natural and synthetic dental materials such as those described above.     

Hybrid plasma CVD of diamond-like carbon (DLC) at low temperatures

Diamond-like carbon coatings have been deposited onto various substrates at 100–150°C using a hybrid plasma assisted chemical vapour deposition technique activated by radio frequency at 13.56 MHz. The coatings have been characterized using a number of techniques including scanning electron microscopy, Raman spectroscopy, thermoanalysis and pin-on-disc wear testing. Results show the films to be diamond like, with the addition of nitrogen (prior to deposition) promoting the formation of crystallites. In addition the condition and type of substrate have been found to have a strong influence on the structural characteristics of the deposited diamond-like films. SEM analysis of diamond-like carbon coatings deposited onto metal matrix composite materials such as Si-Al MMC is reported. The hybrid CVD technology enabled films to be deposited evenly onto the porous MMC structure. Commercially manufactured drills, coated with DLC and titanium nitride (TiN), have been compared to examine their cutting wear resistance characteristics. This revised version was published online in November 2006 with corrections to the Cover Date.     

Chemically vapour deposited diamond coatings on cemented tungsten carbides: Substrate pretreatments, adhesion and cutting performance

Chemical vapour deposition (CVD) of diamond films onto Co-cemented tungsten carbide (WC-Co) tools and wear parts presents several problems due to interfacial graphitization induced by the binder phase and thermal expansion mismatch of diamond and WC-Co. Methods used to improve diamond film adhesion include substrate-modification processes that create a three-dimensional compositionally graded interface. This paper reviews substrate pretreatments and adhesion issues of chemically vapour deposited diamond films on WC-Co. The combined effect of pretreatments and substrate microstructure on the adhesive toughness and wear rate of CVD diamond in dry machining of highly abrasive materials was analyzed. The role of diamond film surface morphology on chip evacuation in dry milling of ceramics was also investigated by comparing feed forces of coated and uncoated mills. The overall tribological performance of diamond coated mills depended on coating microstructure and smoothness. The use of smother films did allow to reduce cutting forces by facilitating chip evacuation.     

Adhesion of diamond coatings on cemented carbides

Precise quantification of the adhesion of diamond coatings on cemented carbide (WC-Co) inserts is important for industrial applications. Adhesion is strongly influenced by the surface roughness, surface reactivity, catalytic effect of Co during diamond chemical vapour deposition (CVD) and by stresses developed in the film and at the bonding interface.

In this work we investigated the adhesion of diamond coatings on WC-Co by using Rockwell-C indentation. Various surface modifications were studied: Co leaching; replacement of Co by Cu; WC-Co decarburization and deposition of Ti intermediate layer prior to diamond CVD. Turning tests with diamond coated inserts for machining of Al alloy were carried out.     

Machining of high performance workpiece materials with CBN coated cutting tools

The machining of high performance workpiece materials requires significantly harder cutting materials. In hard machining, the early tool wear occurs due to high process forces and temperatures. The hardest known material is the diamond, but steel materials cannot be machined with diamond tools because of the reactivity of iron with carbon. Cubic boron nitride (cBN) is the second hardest of all known materials. The supply of such PcBN indexable inserts, which are only geometrically simple and available, requires several work procedures and is cost-intensive. The development of a cBN coating for cutting tools, combine the advantages of a thin film system and of cBN. Flexible cemented carbide tools, in respect to the geometry can be coated. The cBN films with a thickness of up to 2 µm on cemented carbide substrates show excellent mechanical and physical properties. This paper describes the results of the machining of various workpiece materials in turning and milling operations regarding the tool life, resultant cutting force components and workpiece surface roughness. In turning tests of Inconel 718 and milling tests of chrome steel the high potential of cBN coatings for dry machining was proven. The results of the experiments were compared with common used tool coatings for the hard machining. Additionally, the wear mechanisms adhesion, abrasion, surface fatigue and tribo-oxidation were researched in model wear experiments.     

A REVIEW OF THE CHEMICAL VAPOR DEPOSITION (CVD) OF THE REFRACTORY COMPOUNDS OF TITANIUM - A UNIQUE FAMILY OIF COATINGS

This paper is a review of a unique family of refractory compounds comprising the boride, carbide and nitride of titanium. These materials are characterized by high hardness, high wear resistance and chemical inertness. They are obtained as coatings by chemical vapor deposition (CVD) on a variety of substrates such as tool steel, cemented carbides, graphite and many others. Promising deposition techniques are plasma-CVD and metallo-organic CVD. Major applications of these coatings are found in wear, erosion and corrosion protection and in cutting tools     

加工7075航空铝合金用金刚石涂层刀具的制备及其切削性能

采用热丝化学气相沉积(HFCVD)技术在WC-Co8%硬质合金刀具表面制备金刚石涂层,调节甲烷浓度等沉积工艺制备了单层金刚石涂层刀具和微米金刚石涂层(1.2 μm)、纳米金刚石涂层(200 nm)交替多层金刚石涂层刀具。以7075航空铝合金作为切削工件,在无润滑干切条件下测试了单层金刚石涂层刀具和多层金刚石涂层刀具的切削性能。实验结果表明,切削2 h后单层金刚石涂层刀具涂层脱落宽度达到35 μm,刀刃钝化;有多层金刚石涂层刀具的刃型保持完整,涂层无脱落。对单层金刚石涂层和多层金刚石涂层平面样品进行了洛氏压痕实验。结果表明,多层金刚石涂层的脱落面积约为单层金刚石涂层脱落面积的1/5到1/10,进一步说明多层金刚石涂层有更强的抵抗裂纹产生的能力。这些结果表明,金刚石多层结构能提高涂层与基体的界面结合力,延长金刚石涂层刀具的使用寿命。     

Chemical vapor deposition of highly adherent diamond coatings onto co-cemented tungsten carbides irradiated by high power diode laser

The present investigation deals with the definition of a new eco-friendly alternative to pretreat Co-cemented tungsten carbide (WC-Co) substrates before diamond deposition by hot filament chemical vapor deposition (HFCVD). In particular, WC-5.8 wt %Co substrates were submitted to a thermal treatment by a continuous wave-high power diode laser to reduce surface Co concentration and promote the reconstruction of the WC grains. Laser pretreatments were performed both in N(2) and Ar atmosphere to prevent substrate oxidation. Diamond coatings were deposited onto the laser pretreated substrates by HFCVD. For comparative purpose, diamond coatings were also deposited on WC-5.8 wt %Co substrates chemically etched by the well-known two-step pretreatment employing Murakami's reagent and Caro's acid. Surface morphology, microstructure, and chemical composition of the WC-5.8 wt %Co substrates after the different pretreatments and the deposition of diamond coatings were assessed by surface profiler, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction analyses. Wear performance of the diamond coatings was checked by dry sliding linear reciprocating tribological tests. The worn volume of the diamond coatings deposited on the laser pretreated substrates was always found lower than the one measured on the chemically etched substrates, with the N(2) atmosphere being particularly promising.     

HF-CVD of diamond coatings onto Fluidized Bed (FB) treated CrN interlayers

In the present investigation, Fluidized Bed (FB) treatment is applied to pre-treat CrN interlayers onto WC-Co substrates to promote the growth on them of highly adherent diamond coatings. During FB treatment, the CrN interlayers are submitted to high speed impacts of loose abrasives. The action of their cutting edges is able to deeply change the starting morphology of the as-deposited Physical Vapour Deposition (PVD) CrN interlayers, thus promoting the establishment of a highly corrugated surface on which to grow Hot Filament-Chemical Vapour Deposition (HF-CVD) diamond coatings.Growth, morphology, adhesion and wear resistance of the CVD deposited diamond coatings onto the FB treated and just seeded CrN interlayers were looked into and compared to diamond coated WC-Co substrates with the untreated CrN interlayers or pre-treated with a two-step chemical etching (Murakami's reagent and Caro's acid, MC-treatment) or with FB.FB treatment proved to be an effective technique to tailor the surface morphology and roughness of CrN films deposited by PVD-arc technique, and was found to be very useful in improving the adhesion and wear resistance of CVD diamond onto the CrN interlayers.     

Chemical vapour deposition (CVD) diamond coated tools performance in machining of PEEK composites

This paper presents a study about the chemical vapour deposition (CVD) diamond coated tool performance in machining unreinforced PEEK and composite PEEK CF30 (reinforced with 30% of carbon fibres).

The experimental procedure consisted of turning operations, during which cutting forces and surface roughness obtained in composite workpieces were measured.

The obtained results showed a best cutting performance for CVD diamond coated tool in machining PEEK composites, particularly in terms of cutting forces and power consumption, when compared with polycrystalline diamond (PCD) and cemented carbide (K10) cutting tools. This fact is very important due to the minor production costs of CVD diamond coated tools in comparison with PCD tools.