Developments in Surface Treatments and Cutting Techniques

Stephen Day, Technical Services, Goodfellow Metals Limited, Cambridge Science Park, Cambridge CB4 4DJ, UK

Abstract This article discusses diamond.like coatings, true diamond coatings, titanium nitriding, ion implantation, photoetching and laser cutting with emphasis on the impact they may have on design over the next ten years. It is important to have such techniques available for mater/a/s scientists and engineers worldwide, since they are often needed in prototyping or research.

Introduction New techniques are important because they expand the uses of standard materials and allow items to be manufactured to otherwise impossible specifications. Two areas in which techniques are rapidly being improved are surface t reatments and precision cutting.

Surface treatments Surface treatments are normally used to extend the working life of materials. They can be divided into physical modifications, chemical modifications and coatings. Of these three groups great advances are being made in the last two. Titanium nitride and diamond-like films are just two examples of the huge variety of coatings that can be applied by techniques such as physical vapour deposition, and ion implantation

is a surface t rea tment which alters the chemical nature of the outer layers of a material. The properties of the surface t reatments described in this article are com- pared in Table I.

Titanium nitride Titanium nitride (TIN) is one of the best new ultra-hard ceramic coatings. So far its use has been limited to materials which can withstand its application temperature of 350-450°C. Since this is above the tempering temperature of many tool steels, researchers are trying to find ways of applying the coat at a lower temperature. This might be achieved by using ion beams to assist in the application, an approach which could reduce the application temperature to below the tempering temperature of most tool steels and thus

Table I Comparison of surface treatments

Titanium Diamond-Like Real Diamond Ion Nitride Films Fil ms Implantation

2-4 2 0.5 N/A Approximate Coating Thickness (microns)

Normal Application Temperature (°C)

Application Pressure (Torr)

Colour

Hardness (Kg/mm 2)

Heat Resistance

Chemical Constituents

350-450

10-4 _ 10-1

Gold

2000-3000

Good

TiN

Room Temperature

10-5.10-3

Black

Varies with Hydrogen Content

Good

CnH m

650-900

20-100

Transparent

9000

Very Good

C

Room Temperature

<~10-5

N/A

N/A

up to 500°(3

N

20 0261 - 3069/90A010020-05 $3.00 © 1990 Butterworth-Heinemann Limited MATERIALS & DESIGN Vol. 11 No. 1 FEBRUARY 1990

would greatly increase the popularity of the coating. Titanium nitride is applied by evaporating titanium

into a vacuum chamber using either arc evaporation, sputtering or ion beam evaporation <1~. Ionised nitrogen is introduced into the chamber and reacts with the titanium vapour to form a layer of titanium nitride on the material being treated. Coatings between two and four microns thick are normally applied. The coating is gold in colour so it is easy to identify components which have been treated. Apart from its colour, however, titanium nitride follows the original surface and it is used on a wide range of engineering components and tools. It is two to three times harder than hardened high speed steels, resistant to corrosion and oxidation at very high temperatures and has a very low coefficient of friction. Its performance is impressive; it has been shown to increase the life of pistons, drill bits and cutting tools by between three and ten times. Only a fraction of the potential of titanium nitriding has been exploited and it is expected to become increasingly popular in the years to come. Howoever, it does have some fundamental limitations. The coating cannot be used on soft materials because grit is able to deform and then penetrate the surface - causing it to tear. It is also limited by the size of vacuum chambers.

Ion implantation in some cases the gap left by the high application temperature of titanium nitride is filled by ion implantation which does not require the bulk of the substrate to be heated c1~. Furthermore, ion implant- ation alters the structure and composition of the outer layers of a material without causing any change in its dimensions.

The process has a number of useful properties:- It improves resistance to wear and corrosion by three to four times and in some cases by up to twenty times. it is applied at a low temperature and in a vacuum so there is no distortion or oxidation of the material.

* It does not damage polished surfaces. Nitrogen ions are normally used for implantation. The

process involves bombarding a material with accelerated ions which become lodged beneath its surface. The ions occupy spaces caused by defects and displace atoms from the crystal lattice of the material. Some of the ions also react with atoms in the lattice to form Iocalised compounds with new lattice parameters. High concen- trations of implanted ions put the surface layer under compressive stress. This closes up microcracks in the surface and so prevents crack initiation and propagation, improving tensile strength and fatigue resistance. Clos- ing the cracks also prevents gaseous and ionic salts from entering the material and causing corrosion. Further- more, as wear eventually takes place atoms which were trapped interstitially are dislodged and some diffuse deeper into the lattice. This preserves the wear resist- ance to a much greater degree than would otherwise be expected.

Ion implantation is suitable for use on steels, chromium, tungsten carbide, titanium alloys, copper, aluminium, phosphor bronze and even diamond. It cannot, however, be applied to components subjected to high temperatures because the implanted nitrogen

is driven off at temperatures above 500°C. Since ion implantation is a line of sight process there will always be limits to the complexity of the shapes that can be treated, making it most useful for small regularly shaped objects. It is also a rather slow process, being limited by the strength of the ion beam. However, this problem should be partially overcome as stronger ion beams are developed; work at the Harwell laboratory has already produced prototypes with ion beams an order of magnitude stronger than those in use today. Apart from its obvious use in protecting engineering components against abrasion, ion implantation may also have a role to play in medicine. It can be used to extend the life of replacement joints made from titanium alloys. This is important because although titanium alloys have suitable bio-compatibility, weight and strength for use in replacement joints, they are generally not very wear resistant. The life of a hip joint made from a titanium alloy is around 10-15 years. However, this can be increased to 25-30 years by use of ion implanted alloy.

Diamond-like and real diamond films These films consist of an amorphous layer of hydro- carbon polymers in which the proportion of hydrogen varies but can be as high as 50%.

The application of diamond-like films may also have an impact on the life-span of replacement joints. Research at Addenbrooke's Hospital in Cambridge has shown that diamond-like films are compatible with mammalian cells and so are suitable as protective coatings for medical implants <2~. This promises to increase the range of materials which can be used for medical-engineering since the films can keep the implant substrate and the tissues of the body from ever coming into contact.

Diamond-like films are deposited by introducing hydrocarbon gas (butane, propane or acetylene) into a vacuum chamber containing the substrate. An ion beam directed at the substrate causes a surface reaction between it and the gases which results in the deposition of the film. The beam used is almost neutral so materials can be coated without charging effects <2~ and coatings approximately two microns thick are normally applied. The films are extremely hard and insoluble, they absorb visible wavelengths of light but are transparent in the infra-red. These properties can be exploited to provide wear resistant and chemically inert coatings for a variety of materials including titanium, germanium, aluminium and silicon. Diamond like films can be used to provide an anti.reflective layer and because they are transparent to infra-red light they can be applied to computer disks and infra-red windows to protect them against abrasion.

It is possible, however, that diamond-like films will be overshadowed by the advent of real diamond films. The techniques that can coat materials with a layer of real diamond still have their teething problems but research concentrated on solving these is well underway in the United States, Japan and the Soviet Union <3>. One of the more successful techniques of depositing diamond films is by removing the hydrogen atoms from methane gas in a partial vacuum. The carbon atoms are then made to bind the substrate to form a layer of diamond. This process usually requires temperatures of between 650 and 900 o C, although experimentally diamond has

MATERIALS & DESIGN Vol. 11 No. 1 FEBRUARY 1990 21

been deposited at temperatures as low as 300°C. A coating of real diamond offers similar protective qualities to a coating of diamond-like film. However real diamond also offers unmatched transparency, a crystalline structure and a coefficient of friction near that of Teflon. The structure of the diamond allows it to be used as a semiconductor and this is an important factor driving research into diamond coating. Diamond conducts heat ten times more efficiently than silcon so it would be possible to make diamond based computer chips that were more closely packed and therefore faster than present day silicon based chips.

Two products which contain true diamond films are already on the market, a high frequency loudspeaker manufactured in Japan and an ultra-thin diamond window for X-ray sensitive instruments that is made in the USA.

0.346" 8.79 mm

0.012" 0. 305 mm

Diamond z-- film

k ~ ~ - - Support grid

Typical holder (per customer specifications)

Fig I Dimensions of typical ultra-thin diamond X-ray window

Precision cutting Surface treatments are complemented by the tech- niques of precision cutting, perhaps the most important of which are photoetching and laser cutting. These two techniques have increased the range and complexity of possible components. Photoetching is best suited to thin materials and can be used to create complex miniature components. Laser cutting is used for thicker sheets and is somewhat more flexible, but does not lend itself to minute detail as readily as photoetching.

Photoetching Photoetching is popular because it is a cost effective way of producing both simple and complex shapes from thin materials. It offers the two advantages of not sub- jecting materials to mechanical stress or changes in temperature. The technique is becoming increasingly important in producing very small components, reflect. ing the trend towards miniaturization.

The first stage of the process is to program the parameters of the engineering drawing into a graphics computer. A computer controlled scalpel is then used to cut high quality, much enlarged artwork from lamin- ated film. The laminated film consists of an opaque layer on top of a clear one. The scalpel only cuts through the opaque layer, selected pieces of which can be peeled off to leave very clean and accurate edges. The artwork is then photographically reduced to the size required. The metal to be etched is cleaned and coated with a photo-resist solution. When this solution is exposed to light and developed it provides protection against chemical attack. The film of the artwork is placed on top of the photo-resist solution which is then exposed to ultra-violet light and developed. Photo-resist solution which was covered by opaque sections of the artwork and so was not exposed to the light can be washed off to leave just the required shape protected by the developed solution. The material is then sprayed with an etching fluid and the unprotected metal is dissolved away.

Finally the developed photo-resist solution is removed and the dimensions of the component are checked. It is also possible to etch metals from both sides at once by carefully aligning two sets of artwork. This improves the edge tolerances and allows the process to be applied to thicker sheets. Some typical edge tolerances for photoetching are shown in Table I1.

Table I! Typical photoetching tolerances

Material Thickness Tolerance Minimum Possible Apertures

<0.1ram 0.15mm

0.25mm

+ / - O.025mm I

+ / - O.06mm 0.25mm

< 0.5mm + / - 0.1mm 0.5mm

< 0.75mm + / - 0.2mm 1.0mm

Fig 2 Photograph of window (courtesy of CrystaUume, California, USA)

Photoetching is best for materials less than 1ram thick and is not really suitable for use on gold, silver or titanium. However there are exceptions; titanium foil, for

22 MATERIALS & DESIGN Vol. 11 No. 1 FEBRUARY 1990

example, has been successfully etched to make small turbine blades.

Photoetching has been used to cut out hands for wrist watches and valve disks for artificial hearts. The process allows miniaturization which would not other- wise be possible. It is particularly important in the electronics industry where it is used in the production of lead frames. These provide a means where by more than 100 leads can be attached to a single chip and so have allowed for the massive increase in circuit density that has occurred over the last few years. This may become increasingly important if and when diamond based chips come into production since these would allow further increases in circuit density (see above).

Laser cutting Laser cutting offers many of the advantages of photo- etching and can be used on much thicker materials. It can also cost less because artwork does not need to be produced. The parameters are fed direct!y into the laser positioning computer. The technique gives designers flexibility. It has extremely close tolerances and can be used to cut intricate shapes. There is less wastage than with other tooling techniques which makes it ideal for use on exotic and high cost materials. Perhaps its greatest impact, however, will be on prototyping. It is a relatively low cost process and simple programming of the laser can accommodate frequent changes in design.

The effectiveness of laser cutting depends on: focussing the beam; the power of the laser; and controlling the rate at which the beam travels across the material. These can all be controlled independently and some experimentation is required before processing a new material. Ceramics, for example, need to be treated with caution as they tend to shatter if the laser hits an inclusion. Laser cutting normally works by melting or sublimating the material. To achieve a clean finish laser beams are often pointed up at the material being cut so that molten material falls out. Jets of gas are also used to blow away debris and vapour and to cool the cut edge. Oxygen or compressed air is used for ferrous metals and nitrogen or noble gases for non-ferrous metals and for other materials where it is important to reduce oxidation.

Lasers can cut very complex shapes since they can be rotated and positioned on three axes. This contrasts with photoetching which is limited to two dimensional designs. Bouncing laser beams off a mirror allows them to reach awkward places and materials can be cut that are behind glass, in a vacuum or even in a liquid. Laser beams can also be used to drill small holes, where the diameter of the hole is the same as the diameter as the laser beam. However, the depth to diameter ratio of hole produced like this is limited to about 15:1. in very thin foils it is possible to produce holes just one micron a c r o s s .

As in photoetching, laser cutt ing is limited to certain thicknesses of material (Table III). For sheets less than l m m thick photoetching is normally the preferred choice although laser cutting has sometimes been used. This is because photoetching gives a better edge profile as laser cutting can cause thin materials to burn or distort near the cut edge. Laser cutting is also unsuitable

Table III Maximum thickness for laser cutting

Material to be Cut Maximum Thickness

Stainless Steel

High Carbon Steel

Tantalum

Plastics

Mild Steel

Titanium

Aluminium

6mm

9mm

2mm

25mm

9mm

3mm

lmm

for highly reflective materials such as copper, silver or gold.

The development of laser cutting is likely to be dominated by the development of new lasers (4). At present industrial laser cutting uses either carbon dioxide or neodymium:yttrium aluminium garnet (Nd:YAG) lasers. Both of these produce a beam consisting of a single infrared wavelength; the carbon dioxide layer is used where high intensity beams are required and the Nd:YAG laser for low intensity, precision cutting. The development of excimer and free electron lasers will increase the range of applications of laser cutting. Excimer lasers produce beams of ultraviolet light by using the photons released when the bonds in molecules such as XeF are broken. This contrasts with normal lasers which use the photons released when the bonds in molecules such as XeF are broken. This contrasts with normal lasers which use the photons released when electrons return from an excited state to the ground state in intact molecules. An ultraviolet laser could be focussed to a much finer point than an infrared laser because the spot size is determined by the wavelength of the light being focussed. Also the energy of the beam would be near to that of molecular bonds which raises the possibility of using excimer lasers to break molecular bonds without generating excess heat, ie. cold cutting.

In contrast to the lasers so far mentioned the free electron laser can produce radiation of variable wave- lengths. This has advantages since it means that the wavelength of laser beam could be selected to suit the material to be cut. Free electron lasers use the synchrotron radiation that is emitted when an electron beam is accelerated and controlled by magnets. The wavelength generated can be varied by altering the energy of the electron beam or the distance between the magnets.

The use of lasers in surface t reatments In addition to cutt ing materials lasers can also be used in surface treatments for example by providing the activation energy these require. In such cases the surface treatments can be localised by programming the laser to cover only restricted areas. Lasers can also be used to alter the surface itself. In case-hardening they are used to heat the surface to just below its melting point which results in phase transformations which harden the surface. Alternatively, very high intensity beams can be scanned rapidly across the surface of

MATERIALS & DESIGN Vol. 11 No. 1 FEBRUARY 1990 23

metals to produce temperature fluctuations of millions of degrees centigrade per second. This results in the formation of metastable phases with anticorrosive and unusual magnetic properties. Finally, by melting restric- ted areas of the surface, lasers can be used to help produce Iocalised surface alloys, alloy components being dusted into the melted areas.

Conclusion Surface treatments not only protect materials from abrasion and corrosion, they can also give surfaces useful properties. For example, diamond.like films can be used to create an anti-reflective surface and real diamond films can form a semi-conducting surface. As research into surface treatments continues the range of properties that can be given to the surface of a comp- onent is bound to increase. This, combined with im- provements in cutting techniques, will increase the uses of both common and exotic materials.

A c k n o w l e d g e m e n t s The author wishes to thank CrystaUume Inc., Tecvac Ltd. and Ion Tech Ltd. for help during the preparation of this article. Copyright 1989, Goodfellow Metals Limited.

References 1 Saunders, C G S, 'Ion techniques in surface engineering' Metals

and Materials, Vol 4, No 11, p 678-682, ]988. 2 Franks, J, Ng, T L, and Wright, A C, 'Preparation and charac-

teristics of diamond.like carbon films' Vacuum, Vo138, Nos 8-]0, p 749-751.

3 Graft, G, 'Diamonds find new settings' High Technology, April ] 9 8 7 .

4 Bacon, M, 'Lasers. In the heat of the light', Materia]s Edge, 9, p 19-31, 1989.

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