Nano-Structured Alloy and Composite Coatings for High Temperature Applications

Nano-structured materials often possess special properties that materials with identical compositions but ordinary grain size do not have. This paper reports our work on the surface nano-crystallisation and nano-structured alloy and composite coatings. A number of processing methods including magnetron sputtering, thermal spray and pulse electro-spark deposition have been used to produce surface nano-crystalline structure. The compositions and microstructures can be well controlled by using different targets or electrodes, nano-structured composites and adjusting processing parameters. Surface nano-structured coatings can provide special chemical, mechanical and electronic properties such as high temperature corrosion and corrosive wear resistance. It has potential applications such as turbine blades, engine parts for petrochemical, aerospace and electronic device industries. This paper is focused on the study of the interrelations between processing, microstructure and properties. Physical models have been established to explain the effects of nano-crystalline structure on the properties.


Introduction
Nano-and micro-crystal materials often possess special properties that materials with identical compositions but ordinary crystal size do not have.These materials are finding wide applications especially as electronic, photonic, magnetic, and biomedical materials 1 .Nano-and micro-crystal materials are also known to have superior mechanical properties due to the extremely fine grain size 2 .However, it has not been well known that alloy and alloy-oxide composite coatings with nano-and micro-crystal grains have special properties for high temperatures corrosion properties.Nano structures promote selective oxidation, forming protective oxidation scales with superior adhesion to the substrate.Progress has been made in this area as nano-and micro-crystal alloy coatings, oxide dispersive alloy coatings and metal-oxide composite coatings show superior high-temperature corrosion resistance 3 .

Processing of nano-crystal coatings
Nano-crystal and submicro-crystal alloy coatings or thin films can be produced in many ways including chemical vapour deposition (CVD), physical vapour deposition to the level of ~50 nm (Fig. 1b).Zn oxide thin films can also be obtained by oxidation of thin Zn films under a controlled oxygen partial pressure.Nano-structured ZnO grains form clusters of ~100 nm size (Fig. 1c).When ZnO was directly deposited onto a substrate, the grain size was much smaller compared with metallic Zn (Fig. 1d).The microstructure, oxidation state, doping elements and doping levels have strong effects on the electronic properties of the oxide thin films, such as transparency, conductivity and photoluminescence (PL) properties 10 .
Co-deposition and reactive deposition were also carried out using unbalanced magnetron sputter.Co-deposition is to sputter metals from two or more targets, or from a composite target [11][12][13] .Composite target is a plate made of several different materials.Coating compositions can be accurately controlled by adjusting the currents on the different targets, or the constitution of the composite target.Reactive deposition is performed by introducing certain amounts of reactive gases (e.g.O 2 or N 2 ) into the chamber 13 .Alloy-oxide or alloy-nitride coatings can be produced in this way.Fig. 2 shows high-resolution SEM micrographs of co-deposited 310 stainless steel and Al, as well as reactive deposited Al coatings.The co-deposited coating has an average grain size of ~100 nm.The reactive deposited coating shows clusters of ~500 nm, each of which appeared to consist of many small grains.By careful control of the processing parameters, coatings with gradient compositions and microstructures can also be produced.These coatings have compositions gradually changing from interface to surface, resulting in higher concentrations of desirable components near the coating surface, and better interface connection between the coating and metal substrate.These coatings performed very well against high temperature corrosion [11][12][13] .Some novel but simple techniques were also developed to produce nano-and submicro-and micro-crystal coatings.Electro-spark deposition and high-energy, pulse plasma surface treatment are two of the recent developments [14][15][16][17][18] .These techniques use highly concentrated energy to melt the electrode and substrate surface, originally for producing hard and wear resistant coatings 19 .Nanoand submicro-crystal coatings with grain sizes of    produce nano-crystal coatings with strong metallurgical bonding, exhibiting remarkably improved high-temperature oxidation and hot-corrosion resistance 14,16,17 .These techniques also have the advantages of simple/portable equipment and fast/easy operation.Vacuum or protective atmospheres can be applied but are not necessary.These features make the techniques very attractive for industrial applications such as parts repairing and in situ treatment 20 .

Nano-composites and coatings for Ti and Ti alloys
Ti alloys and Ti-Al intermetallics have the advantages of high strength, light-weight and high melting point.They have great potential applications in aerospace and automotive industries due to their excellent mechanical properties at high temperatures and corrosion resistance.However, their SiC and TiC particulate reinforced Ti 3 Al intermetallic matrix composites were fabricated using mechanical alloying and hot pressing or HIPing methods.Fig. 5 shows the microstructure of a Ti 3 Al-20vol.%TiCcomposite after etching.It was found that the oxidation resistance of the composites is significantly improved compared to Ti, Ti alloys and Ti 3 Al intermetallics.Ti 3 Al-SiC showed a rather low mass gain due to the formation of Ti-Si compounds and discontinuous SiO 2 particles.However, the most significant improvement is that all composite samples exhibited excellent scale spallation resistance.While the cast Ti 3 Al suffered severe spallation, no scale spallation could be found on the composites during oxidation tests.It is believed that the small oxide grain size relieves the stresses in the scale effectively, and the complex oxide-metal interface results in strong adherence between the scale and the substrate 21,22 .
A special type of composite, Ti 3 Al(O)-Al 2 O 3 , was prepared by ball milling of TiO 2 with Al and consolidation 23 .It was applied on pure Ti substrate as a coating through thermal spraying.The coatings show superior oxidation and scale spallation resistance at 700 and 800°C up to 400 h exposure, Fig. 6.The enhanced resistance is attributed to the incorporation of Al 2 O 3 into the coating, which plays a multiple role during oxidation.Thick TiO 2 scale with a layered structure formed on Ti metal during oxidation.Cracks tend to develop along the scale, causing detachment (Figs.7a and 7c).On the other hand, the coated material showed very different morphology.The alumina particles served as a diffusion barrier, an inhibitor to crack propagation in the oxide scale, and the connectors between the coating and oxide scale, Figs.7b and 7d.As a result, the coating can increase the application temperature of Ti, Ti and Ti 3 Al alloys from ~650 °C to ~800 °C24 .
Electro-spark deposition technique was also applied to produce aluminide coatings on Ti 3 Al and TiAl intermetallics [25][26] .Isothermal oxidation tests showed that the coated specimens exhibited much lower oxidation rates than uncoated specimens (Fig. 8).The formation of TiO 2 was largely suppressed; and Al 2 O 3 then became the main oxidation product.Furthermore, oxide scale spallation resistance was greatly improved by the nano-structure.No scale cracking or spallation could be observed on the coated specimens during the oxidation tests (Fig. 9).Nano-or submicro-alloy coatings produced by electro-spark deposition, therefore, provide another powerful tool for Ti-Al intermetallics to be used as high temperature structural materials.With nano-crystal alloy coatings, the Al content that is required to form a complete protective oxide scale can be substantially reduced.Experimental results indicated that when the grain size of Ni-20Cr-Al coatings was in the level of ~60 nm, alloys containing ~2 wt.%Al could form a complete α-Al 2 O 3 scale at 1000 °C in air.This concentration is only 1/3 of the required %Al for the Ni-20Cr alloy with normal grain size!Fig. 10 shows the cross-section SEM micrographs of oxidised samples, exhibiting very different oxide scales that formed on the Ni-20Cr-2Al coatings with different grain sizes.

Selective oxidation of nano-crystal coatings
Due to the formation of highly protective α-Al 2 O 3 , the oxidation rate of the nano-crystal coatings is much lower than the coatings with the same composition but a larger grain size.A detailed study showed that the composition and microstructure of the oxide scales are a function of coating grain size.Fig. 11 shows the relationship between the oxide products, coating grain size and Al content, indicating that the selective oxidation of Al is greatly promoted by nano-crystal structure 7 .The promotion effect of nanocrystal grains on selective oxidation is attributed to shortcircuit diffusion of Al through the grain boundaries.
Mathematical models based on simultaneous diffusion through the crystal lattice and grain boundaries have been established to predict selective oxidation and to describe oxidation kinetics.These models showed a good agreement with the experimental results obtained by the authors in other materials and reported in the literatures 7,28 .

Improve oxide scale spallation resistance
Parts working at high-temperatures and corrosive environments rely on the formation of an oxide scale to protect the materials from further oxidation.This oxide scale should be strong, tough and possess good adherence to the metal substrate.A problem many alloys have is that the protective oxide scales do not last long.Scale cracking and spallation take place after certain time at high-temperatures, or during thermal cycles, contaminating the equipment and substantially reducing the service life of the parts.The oxide scales formed on nano-and micro-crystal coatings show a superior resistance to cracking and spallation.Cyclic and long-time oxidation resistance was significantly improved by applying nano-crystal coatings of AISI 310 stainless steel 11 , Ni-Cr-Al [29][30] , FeAl intermetallic compounds 31 , and Ti-Al intermetallic composites 21,23,32 .Fig. 12 compares the cyclic oxidation behaviours of Ni-20Cr-3.5Alas a cast alloy (average grain size 0.4 mm), a submicro-crystal coating (~0.5 µm) and a nano-crystal coating (~50 nm).Both nanoand micro-crystal coatings show good oxide spallation resistance compared to the as-cast alloy.The nano-crystal  coating also showed a lower oxidation rate.
Scale spallation is a complex process influenced by many mechanical and chemical factors.The mechanisms by which nano-crystal structures improve scale spallation resistance are also complex.Several mechanisms can be suggested based on the experimental results.Firstly, nanocrystal structures promote selective oxidation to form more protective oxides.Secondly, the fine-grained coatings and/ or the fine-grained oxide scales often show a fast creep rate at high-temperatures, which can release the stresses accumulated in the scales, and therefore reducing the scale spallation tendency.Thirdly, the oxides formed on nano-and submicro-crystal coatings are pegged onto the grain boundaries to form a complex interface.The so-called "micro-pegging" effect of the inward-grown oxides results in better scale adhesion to the metal substrate.

Summary
This paper demonstrated that nano-and submicro-structured coatings possess superior high-temperature oxidation and corrosion resistance.The high-density of grain boundaries provide fast diffusion paths, promoting the selective oxidation of protective oxide scales.Scale adhesion and spallation resistance are also improved by stress release and micro-pegging effects.Nano-and micro-crystal alloy and alloy-oxide composite coatings have been proven to be a powerful technique to combat high-temperature corrosion.The theories and models developed from this research can be used to design coatings and alloys for high-temperature, energy related applications.

Figure 1 .Figure 2 .
Figure 1.Zinc and ZnO thin films formed with direct oxidation and magnetron sputtering, a) Zn cross-section, b) Zn + 1.2at.%Alsurface morphology showing the effect of Al doping on the microstructure, c) ZnO formed through oxidation of Zn under Po 2 @ 10 -3 atm at 380°C for 24 hrs, and d) ZnO film with DC magnetron sputtering.

Figure 3 .
Figure 3. AFM image of electro-spark deposited coating surface on stainless steel, showing nano-crystal structure produced by rapid solidification.

Figure 4 .
Figure 4. Dispersed oxide particles in the ODS coatings observed by TEM, showing nano-sized, spherical balls of Y 2 O 3 dispersed inside Fe-Cr-Ni-Al-Y 2 O 3 coating.

Figure 5 .
Figure 5. Ti 3 Al-TiC composite fabricated through MA and HIPing: a) etched, shows the TiC particles; b) shows the formation of fine oxides.

Figure 6 .
Figure 6.Oxidation kinetics and scale spallation of Ti with and without composite coating: a-b) 700 °C; c-d) 800 °C.

Figure 8 .
Figure 8. Oxidation of coated and uncoated Ti 3 Al-Nb with ESD aluminide coating One of the most important advantages of nano-crystal coatings is the ability to promote selective oxidation.Engineering alloys rely on the formation of protective oxide films such as Al 2 O 3 and Cr 2 O 3 to resist high-temperature and corrosive environments.Unfortunately, relatively large concentrations of Al or Cr are needed to form a complete Al 2 O 3 or Cr 2 O 3 scale.In the Ni-20Cr-Al alloy system, for instance, > 6 wt.%Al is required to form a complete Al 2 O 3 scale 27 .High Al contents, however, often degrade the mechanical properties of alloys.If the Al content is lower than 6wt.%, complex oxides consisting of Cr 2 O 3 , NiCr 2 O 4 and internal Al 2 O 3 may form, resulting in high reaction rates and poor oxidation resistance.

Figure 11 .
Figure 11.A diagram showing the relationship of grain size, Al content, and oxidation products for Ni-20Cr-xAl at 1000 °C in air.