Analytical transmission electron microscopycharacterization of plasma spray synthesized TiB2Fe coatings

Thin Solid Films, 193/194 (1990) 442-452

442

ANALYTICAL TRANSMISSION ELECTRON MICROSCOPY CHARACTERIZATION OF PLASMA SPRAY SYNTHESIZED TiB2-Fe COATINGS G. L'ESPI~RANCE AND G. BOTTON Centre for the Characterization and Microscopy of Materials, Ecole Polytechnique de Montrbal. P.O. Box 6079. Station A, Montrbal, QuObec H3C 3A 7 (Canada) S. DALLA1RE Industrial Materials Research hlstitute. National Research Council Canada, 75 De Mortagne. Boucherville, Qubbec J4B 6 Y4 (Canada)

Cermet coatings containing TiB 2 crystals dispersed in an iron-based matrix have been obtained by the plasma spray synthesis process developed at the Industrial Materials Research Institute. The main feature of this technique is the combination in a one-step operation of both the synthesis and the deposition process. The resultant coatings are thick and possess a good abrasion resistance. In order to elucidate the fine microstructure of these TiB2-Fe coatings obtained through a rapid solidification process and to understand their good abrasion-wear behavior, a transmission electron microscopy study has been undertaken. Energy-dispersive spectroscopy using an ultrathin window detector and electron energy loss spectroscopy (EELS) techniques, both allowing the detection of relatively light elements, were used. For the range of compositions investigated, EELS was more sensittve for the localization of boron and offered a better spatial resolution. The results have shown that coatings obtained by the plasma spray synthesis process contain TiB2 particles ranging from 10 to 700nm in size dispersed in a matrix of iron and FeTi. The unusual microstructure of these coatings is probably responsible for their good wear resistance.

1. INTRODUCTION To take advantage of the high hardness of titanium diboride and to overcome its brittleness, cermet coatings containing titanium boride particles dispersed in a metallic matrix were developed. To ensure a good dispersion of fine TiB2 particles within the metal matrix an unusual approach has previously been proposed. It consists in synthesizing TiB2 through the reaction of a titanium-bearing alloy with a boron-bearing alloy and in depositing the reacted products onto a substrate by plasma spraying. The thick TiBz-Fe coatings that were obtained by this one-step process exhibited a very good abrasion resistance. In order to understand the good abrasion-wear behavior of these coatings and to elucidate their fine microstructure, a transmission electron microscopy (TEM) study has been undertaken. 0040-6090/90/$3.50

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TEM CHARACTERIZATION OF PLASMA-SPRAYED

TiB2-Fe

443

2. BACKGROUND INFORMATION

The principle for the production ofTiB 2 in an auxiliary iron bath was extended to combine within a coating both TiB2 and iron. Indeed, TiB2 crystals can be synthesized in an auxiliary iron bath by melting a ferrotitanium and ferroboron mixture I. The reaction which occurs between constituents can be represented by the following equations: [e[Fe] +f[FeTi] a[FeTi'l + b[Ti] + c[FeB] ---* ~d[TiB2] +g[Fe] [h[Fe] + i[Fe2B]

[B]/[Ti-I < 2 [B]/[Ti] = 2 I-B]/[Ti] > 2

where a, b, c, d, e, f, g, h and i express mole fractions. The synthesized products depend on the [B]/[Ti] atomic ratio. When the [B]/[Ti-I atomic ratio is less than 2, the reaction products contain also iron and FeTi compounds. Similarly, iron and Fe2B compounds are found in the products when this ratiois greater than 2. Thick coatings were obtained after plasma spraying in air micropellets comprising ferrotitanium and ferroboron 2'3. X-ray diffraction analysis confirmed that the deposit was mostly constituted of TiB 2, iron and a small amount of FeTi because a I-B]/[Ti] atomic ratio of less than 2 was selected to avoid the formation of Fe2B. The abrasion wear resistance of these coatings was measured in accordance with the low stress ASTM G-65 method. The results indicated that the performance of plasma spray synthesized TiB2-Fe coatings approaches that of the high energy WC-Co coatings in spite of their porosity and their low TiB 2 volume content (45 vol.~o) 3. Further development has shown that by increasing the density of these coatings it is possible to improve their performance considerably. All the development of these plasma spray synthesized TiB2-Fe coatings was based on the assumption that by combining both the synthesis and the deposition of TiB2-Fe materials it is possible to form coatings whose microstructure consists of very fine and hard particles well dispersed in an iron-based matrix, this microstructure being the key factor that results in good performance. However, this has not been proved by a suitable characterization method. 3. EXPERIMENTAL DETAILS AND METHODS Macroscopic observations of the coating were carried out with optical and scanning electron microscopy (SEM) on a diamond-polished sample. Energydispersive spectrometry (EDS) was carried out with an ultrathin window detector (UTW) (LZ4 from Link) in the scanning electron microscope at an operating voltage of 15 kV. Finally, Cu K s radiation was used for X-ray diffraction (XRD) analysis of the coating. The specimen for TEM was cut from a section of the coating parallel to the substrate. The thin foil was prepared by argon ion beam milling at liquid nitrogen temperature of a previously dimpler ground 3 mm disc. Thinning was carried out with a 0.2mA beam at 25 ° tilt and 5kV accelerating voltage. Finishing was performed at a 15° beam tilt.

444

G. L'ESPI~RANCE, G. BOTTON, S. DALLAIRE

Observations and analytical electron microscopy were carried out on a J E O L 2000FX instrument equipped with a U T W EDS detector and on a Philips CM30 microscope operated at 300 kV equipped with a parallel electron energy loss (EEL) spectrometer and a Link MCA. Conventional bright field microscopy for imaging and selected area diffraction (SAD) were carried out for grain sizes from 700nm to 10nm diameter in size. EEL spectra were in all cases acquired in diffraction mode. Areas of diameter as small as 20 nm, corresponding to the area illuminated by the electron beam, were analyzed. A relatively large collection angle (/~ = 12mrad) was used in order to minimize the effect of orientation and angle convergence on the specimen (~ = 6.5mrad). Quantification of the spectra was on an IBM RT computer interfaced to the Link MCA. Relativistic cross-sections S I G M A K 2 and SIGMAL24 were used in order to obtain atomic ratios from net intensities. The signal was extracted by background extrapolation (extrapolation interval, 50 eV; integration interval, 50 eV) using the m a x i m u m likelihood estimator 5 and also by the background-independent method of first-derivative (FD) filters 6"7. The latter method was implemented particularly to extract the weak boron signat which, in some areas, was superimposed on a rapidly falling background that it was not possible to model by the conventional A E -R law'*. 4. RESULTS AND DISCUSSION 4.1. Macroscopic observations

The appearance of the microstructure observed in optical microscopy is shown in Fig. 1. A large amount of porosity inherent to the fabrication process of the coating can be seen. Two phases can also be distinguished with some difficulty. The backscattered electron image taken in the scanning electron microscope (Fig. 2) clearly shows at least the three phases and their heterogeneous distribution and size. The low spatial resolution of EDS X-ray analyses in the scanning electron microscope prevented the quantitative analysis of individual phases. The relatively

Fig. 1. Optical micrograph of the coatmg. Fig. 2. SEM image of the coating with imaged backscattered electrons.

TEM CHARACTERIZATION OF PLASMA-SPRAYED TiB2-Fe

445

large (5-10 lam) white particles are iron rich with some titanium while the very dark areas contain mainly titanium with very little iron. Finally, the grey areas contain iron with almost an equal amount of titanium. Boron is difficult to detect even with an UTW EDS detector so that it was observed in only a few analyses carried out at 5 kV (for which the spatial resolution is improved as well as the detection limits for boron). It was not possible, however, from these analyses to identify the general distribution of boron. TEM was necessary to resolve the individual phases. A typical XRD pattern is shown in Fig. 3 together with the identification of the phases-for the measured dspacing of each reflection. In indexing this pattern a large number of phases including FeB, Fe2 B, FeTi, Fe2Ti, TiB 2, Ti3B 4, TiO, TiO2, FeO and iron were considered. The coating was found to be constituted of large amounts of iron and TiB 2 and of small amounts of FeTi and of FeO. No other phases could be identified. %

~ 2'0

0

i

~

3'0

t

i

,0

\

i

•

TiB z

•

Fe

•

reO

•

FeTi

AO

~0

i

i

60

•

•

i

•

70

i

•

do

•

i

9'o

i

1;0

2~ Fig. 3. XRD pattern of the coating.

4.2. Analytical transmission electron microscopy At the scale investigated by TEM, the microstructure still appeared heterogeneous in terms of the distribution and size of the phases. Thus, there were areas (areas A) that were constituted of large particles embedded in a fine matrix while there were other very fine and polycrystalline areas (areas B). Extensive characterization of each area was carried out by SAD and EEL spectroscopy and the results obtained for each will now be described separately. 4.2.1. Areas A Figure 4(a) is a micrograph typical of areas containing relatively large particles, 350-700 nm in size, embedded in a fine matrix. The SAD pattern and the EEL spectrum shown in Figs. 4(b) and 5, spectrum a, respectively were obtained from an individual particle. Indexation of the electron diffraction pattern also shown in Fig.4(b) is consistent with TiB2. In addition, particular attention was paid to extracting the background of the EEL spectrum using the FD method (Fig. 5, spectrum b). The ~i]/[B] ratio obtained after quantification was 0.5, confirming that these large particles are TiB 2.

446

G. L'ESPI~RANCE, G. BOTTON, S. DALLAIRE

O

,77

(a)

(b)

Fig.4. (a) Typical TEM micrograph of areas A; (b) diffraction pattern of a large grain in areas A.

A~

i

'

]

I Fig. 5. EEL spectra of a large particle in areas A: spectrum a, direct spectrum; spectrum b, FD spectrum.

All electron diffraction patterns li'om the surrounding matrix were rmg patterns owing to the fine grain size (50-200 nm) of the phases making up the matrix. Figure 6 shows typical patterns obtained and Tables I and II give the d spacings calculated for the rings and the many spots which are part of other incomplete rings and the (hkl) reflections and d spacings of phases that can account for the observed rings or spots. The relative X-ray intensities are also included but these should be taken only as an indication of the probability of observing the different reflections of the phases in the electron diffraction pattern. The similarity of the d spacings of reflections of many phases makes it difficult to identify unambiguously the phases. However, by careful inspection of Tables I and II, the phases more likely to constitute the matrix can be identified. The second ring in Fig. 6(a) (d = 0.144 nm) can arise from iron, FeTi, TiB and FeB while the first ring (d = 0.208 rim) also includes TiB2, TiaB 4 and Fe2B. Since iron is one of the two

TEM C H A R A C T E R I Z A T I O N OF PLASMA-SPRAYED

TiB 2 Fe

447

,h

(a) (b) Fig. 6. [a~ SAD pattern of the matrix surrounding large particles in areas A; [b) another SAD pattern of the matrix in areas A showing additional spots. major phases detected in the XRD pattern, it is therefore most likely that iron is present in significant amounts in the matrix but it is not possible from these rings to confirm that TiB 2 is present in the matrix. However, spot 1 (d -- 0.263 nm) in Fig. 6(a) and spot 1 (d = 0.267 rim) in Fig. 6(b) can arise from TiB 2, Ti3B 4, FeB, Fe2B, TiB and TizB 5 but, since TiB 2 was the only one of these phases detected by XRD, these spots indicate that fine TiB 2 particles are also present in the matrix surrounding the much larger TiB 2 particles. On the contrary, the two smallest d spacings (0.0947-0.096 nm for reflections 7 and 8 and 0.081-0.0816 nm for reflections 9 and 10 in Fig. 6(b)) can only come from FeTi, TiB 2 and iron. Tables I and II show that the relative intensities of these reflections for iron and TiB2 would be considerably smaller than those of FeTi. It is therefore considered that the mfi.trix surrounding large TiB2 particles is constituted of large amounts of fine iron and TiB2 grains and possibly some,FeI'i, consistent with the XRD results. In areas containing iron and TiB 2 only, the [B]/[Ti] ratio would be 2.0 as for TiB2. Since in all three analyses in Table III [B]/[Ti] is smaller than 2, this indicates the presence of another titanium-rich phase such as FeTi. Finally, it is not possible to exclude the presence of phases such as FeB, TiB, Ti3B4, TizB 5, Fe2B and Fe2Ti. The pre.sence of a reflection at d ~ 0 . 1 8 0 n m particularly supports this. These phases would, however, be present in very small amounts in the matrix and as very small grains (about 50-200 nm). No large areas of the prescursor FeB and FeTi powders were found. 4.2.2. Areas B

Areas B are defined as areas of much finer grains. As seen in Fig. 7(a), the grain size of this area is typically 10-60 nm. The SAD pattern of such an area has a larger number of more complete rings (Fig. 7(b)) than patterns of areas A (see Fig. 6)

448

G. L'ESPI~RANCE, G. BOTTON, S. DALLAIRE

TABLE I .. INDEXATIONOF AN ELECTRONDIFFRACTIONPA'I'rEKNFROMTHE MATRIX.gURROUNDINGTHE LARGETiB2 PARTICLES(F1G.6(a)) Ring or spot number

Measured d spacing

(nm)

Possible phase, d spacing (nm) and relative (X-ray) intensity (%)

1

0.263

TiB 2 Ti3B4 FeB Fe2B TiB Ti2B5

0.262 (60) 0.266 (10) 0.274 (80) 0.256 (15) 0.254 (80) 0.259 (40)

2

0.248

Ti3B4 Fe2B FeB TiB Ti2B5 Ti2B5 FeO

0.2533 (i00) 0.256 (15) 0.238 (80) 0.2543 (80) 0.254 (100) 0.243 (20) 0.249 (80)

3

0.208

Fe TiB2 Ti3B+ FeTi FeB Fe2B Fe2B TiB

0.2027 (100) 0.2033 (I00) 0.210 (75) 0.2097 (100) 0.201 (100) 0.212(100) 0.201 (I00) 0.214(100)

4

0.183

TiaB4 Fe 2Ti FeB TiB Ti2B5

0.1867 (20) 0.1828 (30) 0.181 (80) 0.1863 (56) 0.189 (56)

5

0.144

Fe TiB FeTi FeB

0.1432 (20) 0.1461 (28) 0.1485 (40) 0.148 (60)

b e c a u s e of the presence of a m u c h larger n u m b e r of smaller grains. This helps in the identification of the phases. T a b l e I V gives the m e a s u r e d d s p a c i n g s of the rings a n d the phases that can a c c o u n t for each ring. C o m p a r i s o n with reflections o b s e r v e d for a r e a s A shows t h a t there a r e a d d i t i o n a l reflections seen with d spacings of 0.3248, 0.1577, 0.1523, 0.1381 a n d 0.110 n m b u t the ring at 0.183 n m is a b s e n t (the latter ring i n d i c a t e d the presence of a p h a s e o t h e r t h a n iron, TiB2 o r FeTi). E x c e p t for the reflection with a d spacing of 0.077 nm, all o t h e r a d d i t i o n a l reflections are consistent with the presence of iron, TiB 2 a n d p o s s i b l y F e T i as for a r e a s A. T h e reflection with the smallest d s p a c i n g (0.077 nm) can only c o m e from FeTi, therefore c o n f i r m i n g u n a m b i g u o u s l y the presence of that p h a s e in a r e a s B. All these results are s u p p o r t e d

TEM CHARACTERIZATION OF PLASMA-SPRAYED

TiB2-Fe

449

TABLE II INDEXATION OF ANOTHER ELECTRON DIFFRACTION PATTERN OF THE MATRIX SURROUNDING LARGE

TiB 2

PARTICLES (FIG. 6(b))

Ring or spot number

Measured d spacing (nm)

Possible phase, d spacing (nm) and relative (X-ray) intensity (%)

1

0.267

TiB2 Ti3B 4 FeB

0.262 (60) 0.266 (10) 0.274 {80)

2

0.2155

Fe TiB 2 TiaB4 FeTi F%Ti FeB F%B TiB FeO

0.2027 (100) 0.2033 (100) 0.2117 (90) 0.2097 (100) 0.2199(100) 0.214(100) 0.201 (100) 0.214(100) 0.2153(100)

3

0.180

Ti3B 4 Fe2Ti FeB TiB

0.187 (20) 0.183 (30) 0.181 (80) 0.186 (56)

4

0.126

Fe

0.117 (30)

5

0.122

TiB 2

0.1215 (14)

6

0.125

Ti3B 4 FeTi Fe2Ti Fe2B TiB

0.123 (10) 0.1214(80) O. 125 (60) 0.120(13) O. 124 (40)

7

0.096

TiB 2

0.947 (10)

8

0.0947

FeTi

0.941 (70)

9

0.081

Fe

0.0828 (5)

10

0.0816

TiB 2 FeTi FeTi

0.0832 (6)

0.0859 (30) 0.0795 (100)

TABLE III RESULTS OF THREE ELECTRON ENERGY LOSS ANALYSES OF THE MATRIX OF AREAS A (see FIG. 4)

Atomic ratios

Analysis 1 Analysis 2 Analysis 3

[Bill-r0

[rOIEFe]

Ea]lEFe]

1.6 1.2 1.14

0.46 2.8 0.43

0.67 3.5 0.49

450

G. L'ESPI~RANCE, G. BOTTON, S. DALLAIRE

(a) (b) Fig. 7(a) Typical TEM micrograph of areas B; (b) SAD pattern. TABLE IV INDEXATIONOF AN ELECTRONDIFFRACTIONPATTERNFROMAREASB (FIG. 7)

Ring or spot number

Measured d spacing (nm)

Possible phase, d spacing (nm) and relative (X-ray) intensity (%)

1

0.3248

TiB 2 Ti3B4 FeB

0.322 (20) 0.324 (30) 0.326 (60)

2

0.264

TiB 2 FeB TiB Ti2B 5

0.262 (60) 0.274 (80) 0.254 (80) 0.254 (100)

3

0.248

Ti3B4 Ti2B 5 TiB FeO

0.2533 (100) 0.243 (20) 0.254 (80) 0.249 (80)

4

0.215

Fe 2Ti Ti3B4 Ti3B4 TiB TiB FeB Fe2B FeTi FeO

0~2199 (100) 0.2117 (90) 0.2102 (75) 0.214(100) 0.2161 (64) 0.219(100) 0.212 (25) 0.2097 (100) 0.2153 (100)

5

0.2054

Fe TiB2 Fe2B Fe2Ti Fe2Ti Fe2Ti FeTi

0.2027 (100) 0.2033 (I00) 0,201 (100) 0.2068 (10) 0.2038 (100) 0.1998 (100) 0.2097 (100)

(continued)

TEM CHARACTERIZATION OF PLASMA-SPRAYED TiB2-Fe

451

TABLE IV (continued) 6

0.1577

Fe2Ti Ti3B4 Ti3B4 TiB 2 TiB 2 TiB FeB Fe2B

0.162(10) 0.1518(10) O.1595 (5) 0.1514(20) 0.1613 (14) O.1528 (32) O.160 (80) 0.163(18)

7

0.1523

TiaB 4 TiB 2 TiB FeO

0.1518(10) 0.1514(20) 0.1528 (32) 0.1523 (60)

8

0.1448

FeTi Fe TiB FeB FeB

O.1485 (40) O.14332 (20) 0A461 (28) O.148 (60) O. 146 (20)

9

0.1381

Ti3B4 TiaB4 TiB2 TiB 2 TiB 2 TiB

0.1371 (30) 0.1360 (30) 0.1370(10) 0.1374(16) 0.1311 (8) 0.1362 (72)

10

0.122

Ti3B4 TiB 2 TiB TiB FeB Fe2B FeTi

0.1231 (I0)

FeTi Fe TiaB4 Ti3B4 TiB TiB FeB FeB

0.1214(80) O.I 17 (30) O.I 166 (25) 0.1159(15) 0.I 181 (28) O.1141 (12)

FeTi TiB 2 TiB

O.I052 (30)

11

12

0.1178

0.110

0.1215(14) O.1244 (40)

0.1219 (I0) 0.124(100) 0.120(13) 0.1214(80)

0.117(100) O.112 (60)

0.1104(12) 0.I I01 (16)

13

0.0833

FeTi Fe TiB 2

0.0859 (30) 0.08275 (5) 0.08320 (6)

14

0.0771

FeTi

0.795 (102)

452

G. L'ESPERANCE, G. BOTTON, S. DALLAIRE

by EEL analyses carried out with a 20 nm probe which gave ratios [B]/[Ti] = 1.9, [Ti]/[Fe] = 3.5 and [B]/[Fe] = 6.7, in very good agreement with areas constituted of iron and TiB 2. Other analyses gave ratios I'B]/ETi] = 1.7, ETi]/EFe] = 4.0 and I-B]/[Fe] = 6.9, suggesting the presence of another phase rich in titanium consistent with the identification of FeTi. It is therefore concluded that the very small grains of areas B are grains of iron, TiB2 and FeTi. 5. CONCLUSIONS

This microstructural investigation has shown that the coatings obtained by plasma spraying micropellets comprising ferrotitanium and ferroboron are mostly constituted of two zones: one containing 350-700 nm TiB 2 particles within a matrix of ct-Fe and fine TiB2 particles (50-200nm) and another containing fine grains (10-60 nm) of iron, TiB 2 and FeTi. At the scale investigated by TEM, minor phases of TiB, FeB, TiaB 4, Fe2Ti, Fe2B and Ti2B 5 may also be present. It seems therefore that the presence of very small and hard particles well dispersed within a coating is more important than their volume content as far as the abrasion-wear resistance is considered. However, the investigation should be pursued to determine to what extent the volume content of hard particles could be reduced without lowering the abrasion resistance of coatings in which these particles are dispersed. REFERENCES B. Champagne, S. Dallaire and A. Adnot, J. Less-Common Met., 98(1984) L21. S. Dallaire and B. Champagne, in E. N. Aqua and C. I. Whitman (eds.), Modern Developments in Powder Metallurgy, Special Materials, Vol. 17, Metal Powder Industries Federation, Princeton, N J, 1985, p. 589. 3 B. Champagne and S. Dallaire, J. Vac. Sci. TechnoI. A,3(6)(1985)2373. 4 R.F. Egerton, Electron Energy Loss in the Electron Microscope, Plenum, New York, 1986. 5 M. Unser, J. R. Ellis, T. Pun and M. Eden, J. Microsc., 145 (3) (1987) 245. 6 N.J. Zaluzec and J. F. Mansfield, Inst. Phys. Conf Ser., 78 (1985) 173. 7 N.J. Zaluzec, in Analytical Electron Microscopy 1987, San Francisco Press, San Francisco, CA, 1987. I 2