Tribology International 34 (2001) 203–205www.elsevier.com/locate/triboint
Book review
Handbook of Hard CoatingsEdited by Rointan E Bunshah; Noyes Publications/William Andrew Publishing, New York 2001 (Availablefrom American Technical Publishers Ltd, 27-29 KnowlPiece, Wilbury Way, Hitchin, Herts, SG4 0SX, England,Price £94)
This book is one of over 50 titles in the Noyes Publi-cations Materials Science and Technology Series. Manyof these deal with deposition technologies and coatings.The applications covered in the series range from elec-tronic and semiconducting thin films, to magnetic andmagneto-optical devices, through to the tribological andmetal-cutting applications which are the focus of thepresent book.
Professor Bunshah the Editor of the book, who sadlypassed away in October 1999, has been a major drivingforce in the development of thin film deposition pro-cesses, and, indeed, was Co-Editor of the book seriesand Editor of two of the titles.
The series has become essential reading for physicists,engineers and materials scientists seeking an authoritat-ive overview of specific material science and processtechnologies, especially in fields in which surfaces andinterfaces hold the key to operational effectiveness.
Given this pedigree, the present book has high stan-dards to maintain, not least since (according to the Pref-ace by Dr Stephen Rossnagel) this is the last volume inthe prodigious output of Professor Bunshah and servesas a testament to his contributions over many years.
Professor Bunshah assembled a notable group of con-tributors to write the individual chapters. These beginwith an Introduction and an Overview of Vapour Depo-sition Technologies by Professor Bunshah himself. Thisgives a useful insight into Physical Vapour Deposition(PVD) and Chemical Vapour Deposition (CVD) pro-cesses, including details of the main recent processdevelopments.
Following this is a chapter by Professor Otto Knotekwhich describes thermal spraying and detonation gunprocesses, especially emphasising their historical devel-
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opment and the many process variants now available,particularly for the deposition of hard ceramic and cer-met coatings.
A chapter by Professor Lars Hultman and ProfessorJan-Eric Sundgren discusses the structure/propertyrelationships for hard coatings. This includes mention ofthe growth mechanisms influencing microstructuraldevelopment, particularly for PVD processes. Severalrecent innovative coatings, such as TiAlN, diamond andvarious multi-layer and “superlattice” films are coveredin some detail, especially with regard to methods fortheir characterisation. The influence of process para-meters on film stresses and the consequent impact onfilm properties such as hardness are outlined. Mentionis also made of the factors influencing coating adhesion,such as deposition temperature and substrate microstruc-ture.
The subject of coating characterisation is continuedin the next chapter, by Dr Steve Bull and Dr DavidRickerby. They emphasise the difficulties inherent inthe assessment of hardness and adhesion, and outlinethe means by which indentation and scratch testsshould be interpreted to give useful quantitative infor-mation about coating-substrate systems. They alsodevote a significant part of the chapter to the use ofX-ray diffraction to assess residual stresses and crys-tallite sizes in coatings.
Dr Bharat Bhushan and Dr Bal K Gupta continue theproperty assessment theme in the next chapter, which isentitled “Macro- and Micromechanical and TribologicalProperties”. This chapter, with over 300 references, takessome of the topics mentioned earlier in the book todeeper levels of analysis. For example, they expand onthe “law-of-mixtures” hardness models presented byBull and Rickerby, and also discuss the use of the nano-indentation test for mechanical property assessment.They also extend on the previous chapter’s discussionon the scratch adhesion test, in particular by consideringlow load microscratch testing using a modified nanoind-entation test facility or an atomic force microscope. Theybelieve that the scratches thus produced are more rep-resentative of the damage which can occur in real appli-cations. In essence the test is then a model tribologicalabrasive contact, rather than an adhesion testing device.
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Bhushan and Gupta also discuss friction and wearmeasurements on a macro-scale, using schematic illus-trations. Their discussion then returns to the use of amodified atomic force microscope (AFM) as a frictionforce microscope (FFM), particularly for the acquisitionof microfriction and wear data for magnetic tapes anddisks, and also MEMS devices. The last part of theirchapter concentrates on the friction and wear behaviourof diamond and diamond-like-carbon coatings. Thisincludes an overview of deposition techniques, polishingmethods and process/property relationships. The widevariations in hardnesses, elastic moduli, adhesion, wearand frictional properties of the many types of carbon-based films are examined in some depth by Bhushanand Gupta.
There are two main applications-related chapters inthe book. The first deals with Cutting Tools (by Pro-fessor O Knotek, Dr F Loffler and Dr G Kramer), andthe second covers Non-Cutting Tool Applications (by DrJ Wesley Cox). The former chapter presents wear andfailure mechanisms, and, with particular reference toCVD multi-layer coatings on cemented carbide turningtools, explains how tool lives have been considerablyextended. The authors then describe how PVD coatingsand processes have been developed to cope with the cut-ting operations which CVD coatings are unsuitable for.This provides a revealing insight into the complexrequirements of different types of tools and workpiecematerials. It also highlights the rapid developments inmulti-phase ternary and quaternary PVD coatings tomeet specific needs. This subject is presently goingthrough a rapid evolution and it is therefore perhapsinevitable that the latest work on ultrahard nanocompos-ite coatings is not included in this chapter.
The chapter dealing with hard coatings for uses otherthan for cutting tools is understandably somewhat super-ficial in nature, given the very wide range of coatingprocesses and wear/corrosion situations it seeks to cover.Indeed the main emphasis is on the coatings and pro-cesses rather than applications. Some of the content re-addresses topics covered in other chapters, but there isalso information provided on techniques such as chemi-cal vapour infiltration and electro/electroless platingwhich is not included elsewhere. Wesley Cox also pro-vides data on the influence of PVD TiN coating thick-ness on rolling contact fatigue life.
Professor Bunshah contributes a further chapter(together with Dr C H Stoessel) which deals specificallywith cubic boron nitride (C-BN) and diamond-relatedthin films. Again, the emphasis is on the processes usedto try to synthesise these materials. The word ‘try’ isimportant, since there are clearly still great difficultiesin producing uniform large area coatings on complexshapes at acceptable costs for both of these coatings. Aspointed out by the authors, most reported ‘c-BN’ filmscontain the softer hexagonal boron nitride phase and
unspecified amounts of hydrogen; they also possess lim-ited adhesion to most substrates. The applications poten-tial thus remains uncertain — although the authorsremain upbeat in that regard.
A somewhat similar situation exists for crystalline dia-mond films — though in that case the potentially excel-lent tribological performance of the “diamond-like”amorphous carbon phase and its doped derivativesensure that the range of applications is presently muchwider than for coatings based on boron nitride.
In his final chapter, “Summary, Developments andOutlook”, Bunshah raised the issues which he feltneeded addressing in the immediate future. First hediscusses the current industrial PVD and CVD pro-cesses and their present state of development. Thenhe mentions recent developments in PVD coatings toimprove machining performance. His next sectionconcentrates on the quest for techniques to deposit thehard “alpha series” aluminium oxides at low tempera-ture. Clearly he saw that as an important goal in hardcoating development. This is not unrelated to his nextsection, dealing with the need to develop cutting toolcoatings which will operate under dry machining con-ditions. He also saw a great need to develop decorativehard coatings, having different colours. This is cer-tainly a major growth market for most of the PVDequipment producers. Bunshah also discusses the cur-rent search for ‘superhard’ materials, with a hardnessgreater than 40 GPa, such as cubic boron nitride anddiamond. As mentioned earlier, he confirms that thereremain some significant barriers to the commercial useof these coatings on metal cutting tools. He concludeshis chapter by emphasising the potential of polycrys-talline “superlattice” and other novel superhard coat-ings. These coatings typically have layer spacingsand/or gain sizes in the tens of nanometres range andcan possess extreme hardness properties. Bunshahwrote the chapter when those developing the films sawthe attainment of high elastic modulus as a furtherbenefit of such films. Recent work suggests that thereal benefit of these films, in most applications, willbe the ability to combine high hardness with a rela-tively low elastic modulus, and it is perhaps disap-pointing that those ideas did not emerge before thebook was completed. Nevertheless, overall the bookrepresents a useful source of information on the cur-rent state of the art in hard coatings and the processesused to produce them. Its main emphasis is on thinfilms and the main application area considered is metalcutting. Although there is some repetition and overlap,this is not excessive, and it is useful to observe howdifferent writers address particular themes, such as(for example) the state of play with regard to diamondcoatings. There is some variability in style, such asthe manner in which references are presented. Someof the topics covered in the book are progressing at
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such a fast rate that it would be impossible for a textof this kind to remain entirely topical for an extendedperiod. Nevertheless it manages to convey the essenceof the hard coating field with a high degree of auth-ority and with a quality of content which should enableit to remain one of the significant texts in the field forsome years to come.
Allan MatthewsHull University,
Research Centre in Surface Engineering,Hull HU6 7RX, UK
E-mail address: [email protected]
10 January 2001
