Duplex surface engineering

9.6 Duplex surface engineering

It is well established that thin coatings of PVD TiN can provide a surface with improved tribological properties, e.g. low friction and resistance to wear, but fail under a high applied load. By contrast, deep

Oxidation, corrosion and surface treatment 509 hardened layers produced by energy beam surface alloying can sustain high contact stresses but still

exhibit poor friction properties and high wear rates. It is the combination of both surface engineering technologies that is known as duplex surface engineering. Generally, the sequential application of two, or more, primary surface engineering technologies to produce a surface, or composite surface, with combined properties unobtainable with just one is duplex, or second generation, surface engineering.

Duplex surface engineering has been applied with considerable success to titanium alloys. This employs an oxygen diffusion (OD) deep case hardening via oxygen interstitial solid solution followed by a low-friction, high-wear-resistance diamond-like coating (DLC). To overcome the problem of poor coating adhesion to the substrate a graded TiN/TiC layer is developed between the hardened substrate and the DLC layer.

Problems

9.1 The weight gain for an oxidizing metal measured 47, 117 and 410 g m −2 after 20, 50 and 175 minutes respectively. Determine the weight gain after 500 minutes.

9.2 During service a steel sheet 1 mm thick is protected by tinning. If the tin plate is damaged over 0.5% of its area, generating a corrosion current of 2 × 10 −3 Am −2 , determine whether the steel will rust through in 5 years. (Atomic weight of Fe = 55.9, density = 7.89 Mg m −3 .)

9.3 During the season cracking of brass, the crack growth rate is proportional to crack size at constant stress and to the square of stress at constant crack size. Determine the relationship between the crack growth rate and the stress intensity factor K . If the crack growth rate is 0.30 mm year −1 at a stress of 4 MPa for a crack depth of 0.25 mm, calculate the constant in the relationship.

9.4 Assuming that oxidation of Fe to FeO obeys parabolic kinetics, calculate the weight of metal lost at 600 ◦

C after 1 year if the oxidation constant is 2 × 10 −7 kg 2 m −4 s −1 . What thickness does this correspond to?

9.5 During the electroplating of copper, 1 coulomb of electricity is passed through the cell, which has a cathode area of 10 mm 2 . What thickness of copper is deposited?

9.6 A thin film of radioactive copper was electroplated on the end of a copper cylinder. After a high temperature anneal of 20 hours, the specimen was sectioned and the activity of each section counted. The following results were obtained:

Counts/minute

Distance from plated end (10 −4 m)

Plot the data and determine the self-diffusion coefficient of Cu at the temperature of the experiment.

9.7 Explain the term second generation surface engineering. How are the properties of Ti–6Al–4V alloy improved by a diamond-like coating (DLC)?

9.8 Outline the important features of thermal barrier coatings for nickel-based superalloys.

9.9 Ion implantation followed by diffusion is a modern method of surface enrichment. If nitrogen ions are implanted to a depth of 0.1 µm producing a surface layer of 10 wt% N and a maximum hardness requires a nitrogen content of 0.25 wt%, calculate the time required to produce a

510 Physical Metallurgy and Advanced Materials

C when D = 9.6249 × 10 −12 m 2 s −1 . (Hint: the ‘thin- surface-film’ solution of Fick’s second law is: C 2 α

1 µm hardened layer by diffusion at 1000 ◦

π Dt exp 4Dt , where α is the amount of impurity per unit area present in the initial surface layer, D is the diffusivity, x is distance and t is time.)

x,t = √

−x

9.10 Thermal barrier coating (TBC) systems consist of an outer zirconia-based layer of low thermal conductivity bonded to a superalloy substrate by an oxidation-resistant metallic bond coat. This bond coat is usually based on (Ni,Co)CrAlY or PtAl alloys, forms a protective alumina layer dur- ing service but in so doing prejudices the integrity of the TBC system. In a series of experiments, spallation of the outer ceramic layer occurred after the following exposure periods:

Temperature ( ◦ C)

Time to spallation (hours)

Given that the thickness, h, of the alumina layer on the bond-coat surface grows parabolically with time, t, at a temperature T (in K) over the above range with a rate constant given by

C and why should you treat this prediction with caution?

what would you expect the spallation time to be at 975 ◦

9.11 The strain energy within a thin, flat oxide layer which remains attached to a thick metal substrate during cooling from the oxidation temperature, T ox , is

2 =E 2 −ν , where W ∗ is the strain energy density (J m −3 ) within the oxide, E ox is the Young’s modulus of

ox (1

ox

the oxide (Pa), ν ox =[T ox − T ]), where T is the current temperature, =[α metal −α ox ]), is the difference (in K −1 ) between the linear thermal expansion coefficient of metal and oxide. (i) Derive this equation for W ∗ assuming that the oxide is stress free at the oxidation temperature and that the oxide layer experiences equal biaxial, in-plane strains during cooling.

(ii) Show how the expression for W ∗ can be used to predict the critical temperature drop, T c , to initiate oxide spallation. [Hint: assume that there is no change in strain energy within the alloy substrate when spallation is initiated. Let the effective fracture energy of the oxide/metal interface be γ f , in J m −2 , and the oxide thickness be h, in m.] (iii) Calculate the value of γ f using the following values: E ox

= 380 × 10 9 Pa, ν ox = 0.27, = 8.0 × 10 −6 K −1 ,h = 5.0 × 10 −6

c = 671 K. Why does this value of γ f differ from the intrinsic value, of 2 J m −2 , for fracture of the oxide/metal interface?

Oxidation, corrosion and surface treatment 511

9.12 Although chromia (Cr 2 O 3 ) is thought to be thermodynamically more stable than the spinel FeCr 2 O 4 , demonstrate that it may be possible for iron to reduce chromia according to the

reaction

Fe + 4/3Cr 2 O 3 → 2/3Cr + FeCr 2 O 4 .

(A)

◦ for this reaction is given in J by

G ◦ = −98 115 + 36.0 T , where temperature, T , is in K.

What is the relevance of this calculation? [Hint: assume the activity of a bulk phase is unity.]

9.13 The chromia layer formed by the selective oxidation of an austenitic steel grows parabolically with time, t, and reaches a thickness of 2 µm after the following exposure periods at different temperatures:

Temperature, ◦

C (K)

Time to reach 2 µm thickness

Use this information to obtain an activation energy for the oxidation process and suggest a possible rate-controlling process.

Further reading

Bell, T. (1992). Surface engineering: its current and future impact on tribology. J. Phys D.: Appl. Phys. 25, A297–306. Bunshah, R. F. (1984). Overview of deposition technologies with emphasis on vapour deposition tech- niques. In Industrial Materials Science and Engineering Chap. 12. (edited by L. E. Murr), Marcel Dekker, New York.

Picraux, S. T. (1984). Surface modification of materials – ions, lasers and electron beams. In Industrial Materials Science and Engineering (edited by L. E. Murr), Chap. 11. Marcel Dekker, New York. Shreir, L. L. (1976). Corrosion, Vols 1 and 2, 2nd edn. Newnes-Butterworth, London. Trethewey, K. R. and Chamberlain, J. (1988). Corrosion for Students of Science and Engineering. Longman,

Harlow.

This page intentionally left blank

Chapter 10