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SURFACE ENGINEERING OF TITANIUM COMPONENTS

Objective


The process of plasma nitriding and physical vapour deposition (PVD) coating titanium nitride onto titanium base alloys has been studied and factors affecting wear, fatigue and other properties have been determined. The standard PVD process resulted in poorer contact load carrying ability than plasma nitriding but a new improved PVD process has been developed. Industrial trials have established the benefits of the processes on real components subject to sliding wear under modest loads. The 2 processes are now ready for industrial exploitation.
Laser gas nitriding has been developed and optimized to produce uniform crack free deep hardened layers with good surface finish over extended areas. Excellent resistance to wear in heavy load lubricated rolling/sliding has been demonstrated. Service life may be limited by the onset of cracks leading to spalling, but the process is ready to be applied on a development basis.
A wide range of possible alloying systems has been studied and an electron beam surface alloying treatment, the EBX process, has produced deep hardened layers able to carry heavier loads than laser nitriding and without cracking during wear. The composition and processing parameters have yet to be fully optimized but the process is in many ways superior to other available treatments.
Incorporation of ceramic particles into a titanium surface by laser particle injection has been studied. Both coated and uncoated oxides, carbides and nitrides were used. The process was finally optimized using uncoated particles of titanium carbide. Conditions were defined for coating a broad area with a thick uniform crack free layer containing titanium carbide particles in a tough matrix, and such a coating gave a 10-fold improvement in the wear of titanium by dry abrasion.

The surface of a metal substrate has been modified to give it improved properties. The method is particularly applicable to substrates of titanium, zirconium or hafnium or an alloy based on any of these metals. Titanium is strong and light but has poor tribological properties. A hardened layer from 0.1to 2 mm thick on the surface of titanium of a titanium alloy enhances its wear properties.
A layer of silicon carbide or chromium nitride is painted as a slurry on to the surface of titanium or titanium alloy. The coated substrate was then subjected to surface melting using a scanning electron beam. During traverse of the electron beam, localized melting of the surface layer of the titanium substrate occurred under the silicon carbide layer to form a small pool in which dissolution of the silicon carbide took place with convective mixing. After passage of the traversing beam solidification of the molten pool occurred rapidly as a result of heat transfer to the relatively cool main body of the substrate. The depth of melting could be chosen to suit the intended requirements by appropriate selection of power and/or traverse rate of the high energy beam. The degree of alloying could be chosen by appropriate selection of the ratio of coating thickness to total melt depth. The silicon dissolves in the liquid titanium to produce a substitutional solid solution which may then partially precipitate out upon cooling as titanium silicide. Carbon from the silicon carbide initially dissolves to produce an interstitial solid solution but on cooling precipitates out to form, in addition, a fine distribution of titanium carbide.
The lubricated wear resistance of a treated titanium alloy disc was compared with untreated alloy and hardened steel. With an applied load of 13 kg a steady state wear rate was achieved for the surface alloyed disc for an extended period which was equal to that of hardened steel and one hundredth of that of the untreated alloy.

A method of treating a component made of a metal or alloy has been invented in which the component is bombarded with nitrogen ions in a vacuum system at a temperature below that conventionally used for plasma nitriding. The nitrogen ion bombardment is preferably carried out at a temperature below 850 C and for a period of time in the range 1 to 2 hours. This method allows surface treatment of a component to be carried out relatively quickly.
After nitriding, a surface coating, particularly of a nitride may be formed on the component using physical vapour deposition (PVD). Preferred coatings include titanium nitride, aluminum nitride and titanium aluminum nitride. When coating with titanium nitride by PVD, titanium metal in the vacuum system is vaporized by electron beam evaporation in parallel with continued nitrogen ion bombardment. Treatment is generally carried out for between 20 and 40 minutes at a temperature in the range 350 to 450 C. If desired a further coating of a carbide, oxide or carbonitride may be formed on top of the first coating. Prior to the nitriding treatment the component is conveniently cleaned by ion bombardment with argon atoms in the vacuum system. Such treatment is typically carried out for 30 minutes at a temperature between 300 and 400 C. Nitrogen or titanium ions could also be used for cleaning purposes. If nitrogen is used some plasma nitriding will take place during the cleaning process.
The invention is particularly applicable to the treatment of components of titanium and titanium alloys but may also be used for other materials such as steel and other ferrous alloys.

Extensive data has been obtained on the performance of concrete mixes, coatings and surface treatments both in the laboratory and using naturally exposed reinforced concrete elements. The test results have been analysed in the context of substantial published data and have led to the following conclusions.

Significant (order of magnitude) improvements in the durability of reinforced concrete in salt contaminated environments can be achieved by the appropriate selection of cementing materials and admixtures. In particular, mixes containing either pulverised fly ash (PFA) or ground granulated blast furnace slag (GGBS), in combination with a waterproofing admixture, exhibit properties which are uniquely suited to such exposure conditions. These include low sorptivity, low chloride diffusion and high resistivity.

The ingress of chloride into concrete can be modelled as a 2 stage process; absorption and diffusion. This has been quantified and validated and can be used to predict the time to activation of steel in concrete.

Carbonation occurs at a definable rate, and this model can be used to predict late life carbonation depths from early life results.

Many generic types of coatings provide a barrier to both carbonation and to chloride ingress, and performance can be maintained under natural weathering at least for a period of 3 years.
THE OBJECTIVE OF THIS PROPOSAL IS TO ALLOW MANUFACTURERS TO DESIGN HIGH STRENGTH/WEIGHT RATIO COMPONENTS IN TITANIUM FOR OPERATIONS UNDER CONDITIONS CURRENTLY UNACCEPTABLE. BY ENHANCING THE TRIBOLOGICAL AND LOAD BEARING CHARACTERISTICS OF TITANIUM AND ITS ALLOYS A NEW GENERATION OF COMPONENTS WILL BE AVAILABLE WHICH WILL NOT ONLY BE OF BENEFIT IN SUCH TRADITIONAL AREAS AS THE TEXTILE AND AUTOMOBILE INDUSTRIES, BUT ALSO FOR THE PIPELINE AND AEROSPACE SECTORS.A VARIETY OF SURFACE ENGINEERING TECHNIQUES SUCH AS PLASMA AND LASER THERMOCHEMICAL PROCESSING OFFER CONSIDERABLE PROMISE IN THIS FIELD.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

UNIVERSITY OF BIRMINGHAM
Address
Edgbaston
B15 2TT Birmingham
United Kingdom

Participants (1)

TECHNISCHE UNIVERSITAET CLAUSTHAL
Germany
Address
Adolph-roemer-strasse 2A
38678 Clausthal-zellerfeld