An erosion-resistant coating for titanium and its alloys

Thin Solid Films, 53 (1978) 375-381 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands

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AN E R O S I O N - R E S I S T A N T C O A T I N G FOR T I T A N I U M A N D ITS ALLOYS* S. C. SINGHAL Metallurgy Department, Westinghouse Research and Development Center, Pittsburgh, Pa. 15235 (U.S.A.) (Received April 3, 1978; accepted April 4, 1978)

A pack cementation method is described for forming an extremely hard boride coating on titanium and titanium-based alloys. The coating produced was characterized by metallography, X-ray diffraction, electron microprobe analysis and microhardness measurements. The coating on both unalloyed titanium and Ti-6 wt. ~ A1-4 wt. ~ V alloy consisted predominantly of titanium diboride and was found to be extremely resistant to erosion by high velocity particle impacts.

1. INTRODUCTION

Recent emphasis on the increased usage of coal for power generation has raised the potential for serious material deterioration by erosion in both coal conversion and direct coal utilization systems. This erosion is caused by continual impact of high velocity coal, char or ash particles on the structural components, and it has meant that coatings must be used to achieve acceptable plant operability in these types of energy conversion system. Many components in coal conversion and coal utilization systems have already been identified 1 as requiring erosion-resistant coatings to ensure reliable operation. Structural components in these systems are necessarily fabricated out of a variety of ferrous and non-ferrous materials because of mechanical properties requirements and corrosion considerations, and it is not always possible to form an adherent erosion-resistant coating directly on all these materials. However, a replaceable liner, shield or cladding made of titanium or a titanium-based alloy with an extremely hard titanium diboride surface on it can be employed to combat erosion on many structural components in coal conversion and coal utilization systems. A method of forming an erosion-resistant surface coating on titanium and titanium-based alloys is described in this paper. The ductility and fatigue strength of the base titanium or titanium alloy are usually adversely affected 2 by the coating thermal cycle, and for this reason the boride-coated titanium or titanium alloy is best used as an erosion shield or cladding rather than for the fabrication of the structural component itself. Diffusion of boron into the surface of titanium or a titanium-based alloy results in the formation of titanium diboride (TiB2) which provides an extremely hard * Paper presented at the International Conference on Metallurgical Coatings, San Francisco, California, U.S.A., April 3-7, 1978.

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wear- and erosion-resistant surface. This boron diffusion can be carried out in a gaseous atmosphere by chemical vapor deposition 3, in molten salts by electrolysis #, by a slurry fusion process 5 or by pack cementation. In this investigation, a powder pack cementation method was employed for forming the boride coating on titanium a n d its alloys. This method for boriding is similar to that widely used for the production of corrosion-resistant aluminide and chromide coatings on superalloys 6-s and offers the advantage that structures with complex and intricate geometries can be coated completely and uniformly. 2. THE PACK CEMENTATION PROCESS

In this process, the boride coating is produced via gas phase transport of boron to the surface of titanium (or its alloy) and simultaneous diffusion of this boron into the surface. The experimental arrangement used was essentially identical with that described for the formation of boride coatings on ferrous materials 9. Unalloyed titanium and Ti-6 wt.~o A I ~ wt.~o V alloy specimens, in the form of 2.5 cm × 2.5 cm × 0.3 cm test coupons, were first pickled in a solution containing 20-30 vol.~o nitric acid and 2-3 vol.~o hydrofluoric acid to remove any surface oxide* and were then placed in a powder mixture contained in an AISI 304 stainless steel cylindrical retort. The powder mixture consisted of 50 wt.~o amorphous boron with a particle size of less than 3 gm, 0.75-1.0 wt.~o ammonium bifluoride (NH4F-HF) and the rest alumina powder. In this investigation, ammonium bifluoride acted as the energizer to supply the halogen required to transport boron to the titanium surface. Other halides, however, can also be used as energizers. Furthermore, the amount of boron in the pack can be varied from about 1 to 50 wt.~o. The powders in appropriate quantities were mixed together in a humidity-free chamber and were blended in a rotary blender containing alumina balls for a minimum of 24 h. The retort was filled to the top with the blended powder so as not to leave any air gap and was then sealed with a metallic O ring. The packed and sealed retort was placed in a box furnace preheated to about 1010 °C. The retort was held at this temperature for 48 h; it was then taken out of the furnace and was cooled. When cool, the coated specimens were removed from the powder mixture and were cleaned either in an ultrasonic cleaner or by lightly sandblasting to remove any powder sticking to them. During heating, the following reactions are believed to take place in the pack: 3NH4F.HF(s ) + 2B(s)-~ 2BFa(g) + 3NH3(g ) + 3H2(g)

(1)

2BF3(g) + 2Ti(s)~ 2TiB(s) + 3F2(g)

(2)

2TiB(s) + 2BF3(g )--*2TiB2(s ) + 3Fz(g )

(3)

NH3(g ) + Hz(g ) + Fz(g) ~ N H 4 F - H F ( s )

(4)

Thus, during heating, gaseous boron trifluoride (BF3) is formed. When this gas encounters the surface of the titanium specimen to be coated, it deposits boron onto the specimen since the activity of boro.n is lower at its surface than in the gas phase. * Titanium is a highly reactive metal and forms a thin oxide layer on its surface when left exposed to air. This oxide layer hinders the diffusion of boron into the surface during the pack cementation process.

EROSION-RESISTANT COATINGS FOR

Ti AND

ITS ALLOYS

377

The boron thus deposited diffuses into the titanium specimen, forming the desired borides (TiB and TiB2). The inward diffusion depletes the concentration of boron and hence lowers its activity at the surface of the specimen so that deposition of boron from the gas phase continues. The energizer NH4F.HF is continually regenerated within the pack according to reaction (4) and is thus not consumed in the coating process. The same powder mixture can therefore be used several times in the pack. The temperature and the time a pack is held at that temperature determine the extent of diffusion of boron into the surface of titanium or its alloy and hence the thickness of the coating formed. A heating time of 48 h at 1010 °C was found to produce a coating about 25 ~tm thick on unalloyed titanium and a coating about 20 lain thick on Ti-6 wt.~ A I ~ wt.~o V alloy. 3.

CHARACTERIZATION OF THE COATING

The coatings produced on both unalloyed titanium and Ti-6 wt.~oA1-4 wt.~ V alloy were continuous, smooth and free of any surface irregularities. The metallographic cross sections of the coated titanium and Ti-6 wt.~ AI-4 wt.~ V alloy are shown in Fig. 1, and reveal that the coatings are uniform in thickness and free of any porosity or microcracks. The coatings on both materials were identified by X-ray diffraction analysis as predominantly TiB 2 with only occasional traces of the TiB phase. The TiB phase could usually be seen in the metallographic cross sections of the coating at the coating-substrate interface, as is evident in Fig. 1. The protrusion of the coating into the substrate, also evident in Fig. 1, results in excellent adherence of the coating to the base material. The electron microprobe scans through a typical boride coating on unalloyed titanium, shown in Fig. 2, confirm that the coating consists predominantly of TiB 2 with only a very thin region of the TiB phase at the coating-substrate interface. The microhardness of the coating was determined using the Knoop indenter with 50 g load. The hardness of the predominant TiB 2 phase in the coating varied between 2800 and 3450 KHN. Titanium diboride is thus one of the hardest materials known, and this extreme hardness is responsible for providing protection against erosive damage by high velocity particle impacts. 4.

EROSION RESISTANCE OF THE COATING

The erosion resistance of the boride coating on unalloyed titanium was determined and compared with that of other hard materials by sandblast erosion tests. In these tests, pressurized air propelled highly abrasive silica powder with a particle size of 75 ~tm through a nozzle at a high velocity; the powder then impacted on the specimen to be tested under the following conditions: nozzle diameter nozzle air pressure nozzle-to-target distance angle of impingement

4.76 mm 0.17 MPa 3.81 cm 45 °

The samples were analyzed for weight loss as well as for penetration depth using a

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SINGHAL

fiB 2 Ti B

Ti

Ti B2 TiB

Ti-6 A~-4 V

Fig, 1. Metallographiccross sections showing the boride coating on (a) unalloyed titanium and (b) Ti-6 wt. '),~,A 4 wt. o/,,V alloy. !Magnification, 400 x .) point micrometer. The results of these sandblast erosion tests for boride-coated titanium and several other hard materials are summarized in Table I. It is clear from these results that the TiB/ coating on titanium outperforms other hardfacing materials by several orders of magnitude under severe erosive conditions. The excellent erosion resistance of the boride coating on titanium and its alloys was further confirmed by particle erosion studies in a shock tube. In this shock tube a boride-coated titanium specimen was impacted with about 18.5 g flyash at a particle velocity of 120-180 m s- 1 and an impingement angle of 40 ° for 150 ms. The particle size distribution of flyash used for these tests was as shown in Table II. Before the test, a narrow section of the coated specimen was masked with vinyl tape to facilitate a comparison of the damage done by flyash impact on the exposed and the protected areas of the specimen. After the flyash impact, the specimen was examined with a scanning electron microscope. The scanning electron micrograph of the exposed and the protected regions of the boride-coated titanium specimen after the flyash impact is shown in Fig. 3. The exposed TiB2 surface remained identical with the protected surface and did not

EROSION-RESISTANT

~ ~

COATINGS

FOR

Ti

379

AND ITS ALLOYS

SpecimCurrent en ,~iB TitaniuSubst m rate..

i TB i2 ~

Fig. 2. Electron microprobe scans for boron and titanium through the boride coating on unalloyed titanium. TABLE I S A N D B L A S T EROSION TEST RESULTS WITH SILICA ABRASIVE

Material

Weight loss per Ib of abrasive (mg)

Penetration per lb of abrasive (pro)

TiB2-coated titanium Hard chrome-plated 304 steel Stellite 6B Tungsten carbide on 304 steel

0.5 79.8 101.2 147.8

7.6 200.6 266.7 368.3

TABLE II PARTICLE SIZE DISTRIBUTION OF FLYASH USED IN SHOCK TUBE EROSION TEST

Particle diameter (~tm)

Weight percentage

0-44 44-125 125-150 150-425 + 425

30.2 56.5 4.7 7.5 0.2

s h o w a n y m i c r o s t r u c t u r a l d a m a g e . In contrast, deep craters were f o r m e d on o t h e r h a r d f a c i n g materials, i n c l u d i n g h a r d c h r o m e p l a t e on stainless steel a n d Stellite 6B, u n d e r the s a m e conditions. T h e severity of the e r o s i o n d a m a g e on Stellite 6B is illustrated by its m i c r o g r a p h , also s h o w n in Fig. 3. T h u s it is clear t h a t a t i t a n i u m d i b o r i d e c o a t i n g o n t i t a n i u m or its alloys p r o v i d e s excellent p r o t e c t i o n a g a i n s t p a r t i c u l a t e e r o s i o n a n d is thus ideally suited

380

s.c. SINGHAL

A Exr 7

FI3 In )tected Area

Fig. 3. Scanning electron micrographs of the surface after flyash impact on (a) boride-coated titanium and (b) Stcllite 6B.

for use on structural components which are subjected to severe erosive environments in coal utilization and coal conversion systems. However, as mentioned earlier, the mechanical properties of the base titanium material are adversely affected by the coating thermal cycle and for this reason the boride-coated titanium (or titanium alloy) is best used as an erosion shield on structural components. 5. CONCLUSIONS

A boride coating consisting predominantly of TiB 2 can be formed on the surface of titanium and its alloys by a pack cementation process in which boron is transported to the surface t o b e coated via boron trifluoride gas generated within the pack. Such a boride coating is extremely hard and provides excellent protection against high velocity particle erosion. In the form of a protective shield, boridecoated titanium can be used to combat severe particle erosion on many structural

EROSION-RESISTANT COATINGS FOR Ti AND ITS ALLOYS

381

components in both coal conversion and direct coal utilization systems for power generation. ACKNOWLEDGMENTS

The author would like to thank D. A. Mitchell for data on sandblast erosion tests, E. F. Sverdrup and his associates for shock tube tests and R. W. Palmquist for electron microprobe analysis. The technical assistance of J. A. Fraino is also gratefully acknowledged. REFERENCES 1 H.E. Frankel and S. J. Dapkunas, Thin Solid Films, 45 (1977) 211. 2 M. Levy and J. L. Morrossi, Erosion and fatigue behavior of coated titanium alloys for gas turbine engine compressor applications, Army Mater. Mech. Res. Cent. Rep. A M M R C CTR,76-4, 1976. 3 T. Takahashi and H. Kamiya, J. Cryst. Growth, 26 (1974) 203, 4 D. Schlain, F. X. McCawley and G. R. Smith, Electrodeposition of titanium diboride coatings, Bur. Mines Rep. R1-8146, 1976. 5 V.S. Moore and A. R. Stetson, Development of erosion-resistant claddings for helicopter rotor blades, Arm), Mater. Mech. Res. Cent. Rep. A M M R C CTR 76-9, 1976. 6 G.W. Goward and D. H. Boone, Oxid. Met.. 3 (1971) 475. 7 S.J. Grisaffe, in C. T. Sims and W. C. Hagel (eds.), The Superalloys, Wiley, New York, 1972, p. 341. 8 S.R. Levine and R. M. Caves, J. Electrochem. Soc., 121 (1974) 1051. 9 S.C. Singhal, Thin Solid Films, 45 (1977) 321.