Filtered Cathodic Arc PVD

Cathodic-arc evaporation is a relatively simple PVD technology that produces a large flux of highly ionized vapor, valuable for depositing hard, dense, and well adhered industrial coatings. Cathodic-arc evaporation also produces macroparticles (MPs) that create defects in the films, relegating this technology to applications that are mostly insensitive to these defects, such as cutting tool coatings. Many methods have been tried over the years to filter out MPs. Although more or less successful at reducing MPs, all of these filtered cathodic-arc (FCA) sources also reduce the coating rate and area to such an extent that they are mostly relegated to the laboratory or to applications needing only extremely thin films over small areas. FCA technology is also typically quite complicated, bulky and expensive.
The present convention in FCA, the curvilinear FCA, borrows from fusion research to bend the ions through a bent tube using a magnetic field. MPs, unlike the ions, are unaffected by the magnetic field and travel in straight lines, getting caught on the walls of the tube and are thereby prevented from reaching the part to be coated. The main problem, also present in fusion technology, is that the ions are imperfectly confined and mostly don’t make it out the filter, which explains the low deposition rates inherent to conventional FCA. Curvilinear filters are also complicated to operate and typically quite large, sticking some distance out of the side of the vacuum chamber. Low deposition rates over small areas (magnetic restoring can increase coating area, but adds even more complication), difficult operation, bulky size, and high cost have prevented wide-spread adoption of filtered-cathodic-arc (FCA) technology, despite the many advantages of ion deposition.

PVD Diamond Like Carbon Thin Films

Continuing our discussion of diamond thin films, we move on to PVD diamond-like-carbon (DLC) films. The main advantages of PVD DLC over CVD diamond are lower temperature deposition (room temperature versus 400C at best for CVD), lower cost typically and a benign environmental footprint. The best PVD DLC is deposited using cathodic arc and can be 3 times harder than sputtered DLC. Cathodic arc produces carbon ions which, with careful substrate bias control, allow the ideal energetic conditions for optimizing diamond, sp3 bonding in the growing film. High sp3 bond ratio correlates with high hardness, up to about 90 GPa, or about the same hardness as CVD diamond (http://www.mrs.org/s_mrs/sec_subscribe.asp?CID=2574&DID=118953&action=detail). Some DLC films have been reported to be even harder than natural diamond; natural diamond nano-indentors can break during hardness measurements.
The main drawback of PVD versus CVD diamond is the difficulty in growing thick films due to compressive stress. CVD "diamond-sheet" films can be 50 microns thick, compared to about 2 microns maximum for the best PVD DLC films. Numerous process modifications have been developed for the relief of stress, including post deposition annealing and substrate high voltage pulsing, but no one has yet brought a high sp3 DLC film to market that is as easy to widely implement as other more standard PVD films, such as titanium-nitride.

Physical Vapor Deposition to Generate $10 Billion in 2008

World physical vapor deposition industry will be worth an estimated $9 bln in 2007 and $9.9 bln in 2008. It should reach $16.7 bln by 2013, a compound annual growth rate (CAGR) of 11% over the 5-year period, BCC Research says. The market is broken down into applications of PVD equipment, materials deposited and services. Of these segments, the PVD equipment will remain the largest market as shipments grow at CAGR of 9.6% to reach an estimated $7.1 bln in 2008 and then increase to $11.9 bln in 2013, at a CAGR of 10.9%. Materials deposited hold the second largest share of the market. Worth an estimated $1.3 bln in 2007, this segment is expected to be worth $1.5 bln in 2008 and $2.7 bln in 2013, a CAGR of 12.4% over the forecast period. The value of services will increase from $1.2 bln in 2007 to $1.3 bln in 2008, and will increase at a CAGR of 9.9% to reach $2.0 bln by 2013.

DLC and Nanotube "Nanomattress"

In most mechanical systems, friction or vibration are often considered to be negative attributes because they results in wear and unnecessary energy dissipation. Tribological issues such as friction, wear, and vibration have always plagued developers of small-scale mechanical devices. As devices get smaller and even reach the nanoscale, this problem becomes more acute due to the extreme surface-to-volume ratios these devices have. In nanomachines damage to even a single atom layer could mean disaster. Nanotechnology researchers basically have two ways to address this problem: they either could apply traditional tribological methods by trying to integrate dampers and low-friction materials with their nano- and microdevices - which becomes increasingly complex and costly at the nanoscale - or they could try and develop intrinsic damping materials that have hard, low-fiction surfaces to lower wear yet still maintain high compressibility and elastic properties to provide resistance to vibrations and shocks. Finding materials that address these problems individually is not difficult but fabricating a structure that combines all of them is nearly impossible because of the conflicting nature of these attributes. However, in what researchers have dubbed a 'nanomattress', a unique structure containing aligned carbon nanotubes (CNTs) covered with a hard layer of diamond-like carbon (DLC) results in a carbon-based composite material with outstanding mechanical properties.

The hybrid film was prepared by first growing a dense, vertically aligned, multi-walled CNT network (CNTs are ca. 7 µm long with 200 nm diameters) by thermal chemical vapor deposition (CVD) on a conductive silicon substrate. The prepared sample was then placed in a patented, off-plane, double-bend, filtered cathodic vacuum arc PVD system with substrate pulse biasing to deposit a 5 µm a-D film on the surface of the CNTs. In order to confine the coating to the top of the CNT film and prevent it from seeping through, a negative substrate pulse bias was used. Because of the negative voltage pulse on the CNT substrate, the high-energy positive carbon ions were attracted primarily to the tips of the CNTs, forming a-D nanospheres. As these spheres increase in size, they coagulate into a thick, solid a-D film on top of the CNTs. The top layer that formed in this manner adhered very well to the tips of the CNTs, as the carbon ions formed strong covalent C–C bonds to them. With a hard, solid top, the nanomattress is now capable of distributing the forces applied to it uniformly to the underlying CNTs, which act like the springs of a mattress.

This information was derived from: http://www.nanowerk.com/spotlight/spotid=3453.php

Tribological Coatings, Past, Present and Future

Tribology, the study of the friction and wear of materials, comes from the Greek word, tríbein, meaning to rub. The first tribological coating for controlling friction and wear was titanium carbide (TiC), introduced in 1969 on cemented carbide cutting tool inserts using chemical vapor deposition (CVD). The problem with the CVD process was that the substrate temperature during deposition was about 1000ºC so that CVD could not be used to coat high speed steel tooling, which is softened at those temperatures. To overcome this obstacle, workers began using physical vapor deposition (PVD) techniques that provide ion bombardment of the growing film, resulting in good film adhesion and densification of the film.

The first commercially successful PVD hard coating was titanium nitride (TiN). Balzers deposited it with their low voltage electron beam process, Ulvac with their hot hollow cathode process, and Multi-Arc with their cathodic arc process. Since the cost of the arc coating equipment was less than that of competing deposition processes, the cost of the arc coatings was lower, and the use of cathodic arc deposited hard coatings became widespread.

Initially, sputtering was not used for the commercial deposition of the tribological films because the quality of the films did not equal that of films produced by low voltage electron beam or cathodic arc methods. This situation was significantly improved with the introduction of closed-field unbalanced magnetron sputtering that provided for a higher degree of substrate ion bombardment during deposition.

One of the early themes in PVD tribological coatings was that high hardness was the most important property. It is true that a coating used for a machining application must be hard, but it is now understood that a coating should be both hard and ductile if it is going to perform well in a tribological application. Superlattice, multilayer, nanostructured, MAX phase, and carbon nitride (a form of diamond-like-carbon (DLC) film) coatings have succeeded in achieving a measure of success in providing both hardness and ductility.

Many DLC films are produced by PVD techniques including cathodic arc, filtered cathodic arc, sputtering, reactive sputtering, and low pressure CVD, and plasma assisted or plasma enhanced CVD processes. The hardness of DLC films covers the range from hard to superhard with hardness of 20-80 GPa. Whereas hard coatings such as titanium nitride, titanium aluminum nitride, and multilayer films have been used very successfully for tooling applications, the DLC films have been very successful where low friction and low wear are needed such as on gears and bearings.

There are two areas that will be very important for the future of tribological coatings. The first of these is nanolayered and nanocomposite coatings, which have already had a major impact on tribological coatings. Another area that should have a major impact on tribological coatings is the use of ionized PVD. Here the recent introduction of high power pulse magnetron sputtering (HPPMS) is used to provide a high degree of ionization of the sputtered material, improving the quality of the coatings and allowing the deposition of films that previously could not be done with conventional sputtering.

Most of this information was obtained from an article by William Sproul on page 46: http://online.qmags.com/SVC1007/

PVD Hard Coatings at the Cutting Edge

The commercial acceptance of hard coatings for cutting tools is driven by demands on machining productivity, environmental mandates, and increased usage of new difficult-to-cut materials. Improved cutting performance is derived from synergies of machine tool system and cutting tool development. The latter strives for an optimized combination of tool material, hard coating and cutting edge geometry.

The integration of hard coatings in cutting tools has reached a mature stage after more than three decades of proven performance and productivity benefits in industrial metal cutting. Chemical vapor deposition (CVD) was the first technology used, which advanced from single layer to current multilayer types combining TiC, TiN, TiCN and Al2O3. Some 25 years later it is evident that novel physical vapor deposition (PVD) compositions have surpassed the limited set of available CVD coatings. The relative economics are debatable but it is accepted that total life cycle consideration and environmental friendliness favor PVD technology. Succeeding generations of PVD hard coatings have become commercially available, notably TiCN, TiAlN and AlCrN, with demonstrated success in expanding areas of application.

Intensive development of the superhard coatings diamond (diamond-like-carbon DLC) and cubic boron nitride (CBN) peaked in the 90’s. It seems that an insurmountable technical barrier – extremely high residual stress during deposition – has not been overcome and still prevents successful commercialization.

In mulitlayer coatings, particular properties suitable to the cutting wear conditions can be engineered by creative combinations of coating layers. For example, AlCrN-TiSiN performs well in high speed carbide drilling of 1045 steel. Although AlCrN has low thermal conductivity, it has been shown that the multilayer construction with TiSiN which has high thermal conductivity effectively raises the composite conductivity, minimizing the heat concentration at the critical drill outer corner. This apparently accounts for better performance of certain multilayer coatings with carbide drills. Another example is the TiAlN-WC/C coating designed for drills used for dry machining.

The figure: coating selection guide based on field performance and commercial acceptance.

An excellent historical overview of the development of PVD coatings leading up the latest innovations written by Dennis Quinto. Jump to page 17 for more... http://online.qmags.com/SVC1007/