Friction and wear of boride ceramics in air and water

Friction and wear of boride ceramics in air and water

Wear, 169 (1993) 63 63-68 Friction and wear of boride ceramics in air and water Kazunori Mechanical Akira Asahi Umeda Engineering Mitsui Glass ...

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Wear, 169 (1993)

63

63-68

Friction and wear of boride ceramics in air and water Kazunori Mechanical

Akira Asahi

Umeda Engineering

Mitsui Glass

(Received

and Yuji Enomoto Laboratory

and Kazuo

Co., Ltd,

November

Hazawa-cho,

Namiki

l-2,

Tsukuba-shi

305 (Japan)

Yokohama

221 (Japan)

Mannami Kanagawa-ku,

3, 1992; accepted

March 18, 1993)

Abstract The tribochemical response of hot-pressed boride ceramics, ZrB,, B4C, ZrB,+B.,C and ZrB,+ B,C+SiC, has been investigated in air and in de-ionized water. In dry air, the coefficients of friction for all materials tested were about 0.9-0.95, while in humid air of relative humidity (r.h.) 95% they ranged from 0.2-0.3, except for that of ZrB,, which was 0.4. Hertzian fractures were noted on the worn surfaces of monolithic boride and ZrB,+B,C composite, but not on those of Sic-toughened boride or Sic-toughened ZrB,+B,C. In water, ZrB, as suggested by the following reaction: ZrB,+ 10H20 underwent very severe tribochemical degradation, --+Zr(OH), + 2H,BO, + 5H,, while B4C, ZrB, + B,C and ZrB, + B,C + SIC underwent less tribochemical degradation, and showed lower specific wear rates than ZrB, in water.

1. Introduction

Boron-based ceramics, such as ZrB, and B,C, are very hard materials with high electrical conductivity and high melting points, which are less wettable with metals. In their monolithic form, however, their toughness is low. In order to obtain highly toughened, electromachinable ceramics, double or tertiary composite boron-based ceramics have been developed using a hotpressing method, i.e. ZrB, + B& ZrB, + B,C + Sic [ 11. In order to determine whether these materials are suitable for tribological applications, a series of sliding experiments was performed to examine their tribological response in the types of environment in which applications are likely. For example, we at first thought they might have properties suitable for sliding components operating at high temperatures, such as hot extrusion dies. Their tribological response at elevated temperatures up to 1000 “C has previously been investigated in air. An interesting observation was that the friction coefficient of these materials, when rubbed against themselves, was as low as 0.1 at temperatures above 650 “C, since an oxidized product B,O, acted as a lubricant. These results have been reported elsewhere in detail [2]. One other kind of interesting application of ceramics is their use as sliding components operating in ambient atmosphere or in low-viscosity fluids, such as water. There are many applications: for example air slides,

0043-1648/93/$6.00

dies, mechanical seals, valves, and journal bearings for the cooling systems of atomic power plants, as well as in ceramic gas turbines and applications where corrosion-resistant tribomaterials are needed. Sliding tests of silicon-based ceramics, S&N, and Sic, in water have been reported [3-51. No reports have been found on boride ceramics in water. This paper describes a series of sliding tests of boronbased composite materials in air of varying humidity and in de-ionized water.

2. Experimental

2.1. Test specimens ZrB,, B,C and its compounds were synthesized by a hot-pressing method at Asahi Glass Co. Ltd at a temperature of 2100 “C for 1 h and at a pressure of 34.3 MPa in argon. Their composition and physical properties are shown in Table 1. Before the sliding tests, their test surfaces were finished with paste containing diamond powders (= 1 pm) using a tin lapping plate for blocks and a lapping cloth for pins. The surfaces were then thoroughly washed with acetone using an ultrasonic cleaner, and finally dried in air. Their average surface roughnesses are shown in Table 2.

0 1993 - Elsevier Sequoia.

All rights reserved

TABLE

1. Properties

Material

of specimen

materials ------..--___I_--_

Composition (wt.%)

.__.__.__~.._ .___._ ._._ _ Density

Vickers hardness {kg mm ‘i

(g cm -+ )

Fracture toughness (MPa rnIrz)

-_--___-._-._-.I-_xII___

ZrB, BK ZrBp f B&I ZrBt + B, t SiC

ZrB,:lOO B,C:lOO ZrB&O, B&z50 ZrB,:46, B&46,

SK?8

5.9.5 2.52 3.57 3.56

-__--.

1600 3100 ._

4.2 3.7

2700

5.0

Bending strength (MPnl

- ...I .._ _ _--...-

.--..__--_

TABLE

2. Average

Material

surface

roughness

of test specimens

.._...

570 3.75 f&i

--__

and then decreased at the same rate. The configuration and the specimen size are shown in Fig. l(b).

Surface roughness

ZrB, B,C ZrBZ + BqC ZrB, + B&J f Sic

R G.4

R,,

0.03 0.008 0.01 0.02

0.65 0.16 0.33 0.46

(a) Fig. 1. Configuration of specimens humid air; (b) in water.

b-4

3. Results and discussion 3.1. Friction properties in air Figure 2 shows the coefficient of friction, f, at a steady state over 100 cycles at each relative humidity, as a function of the relative humidi~ in air. For all test specimens, the coefficient of friction was 0.95 or higher in dry air (relative humidity < 10%) and decreased with increasing relative humidity. At relative humidities above 70%, the value off fell to between 0.2 and 0.3 except in the case of ZrB,, where the lowest value off was 0.4. Figures 3-6 show optical micrographs of the sliding damage in air at typical relative humidities. Many cracks developed normal to the sliding direction, and typical examples are shown in Figs. 3(a) and (b), Fig. 4(a) and Figs. S(a) and (b). These cracks are attributed to Hertzian fractures occurring on the sliding surface

(4) for the sliding test: (a) in

2.2. flying tests in air Sliding tests of these materials were made using a reciprocating type pin-on-block machine similar to that reported elsewhere [6]. As shown schematically in Fig. l(a), the pin size was 4x 15 mm with a hemispherical tip of radius 2 mm, and the block size was 10 X 15 X 6.5 mm. Both pin and block were made of the test material. The tests were made at a load of 7.8 N and at a sliding speed of 1.5 mm s- ’ in a glove box in which the relative humidity was increased in steps from 10% to 95%, with 100 sliding cycles at each step. 2.3. Sliding tests in water The sliding tests in de-ionized water were made using a ring-on-disk machine similar to that reported elsewhere [7]. The tests were made at a load of 147 N. Every 15 min the sliding speed was increased in steps from 12.5 to 800 mm s-l, each step doubling the speed,

0.8 0.6 -

,I20

40

60

80

100

[email protected] humiidity ($X0)25 “C Fig. 2. Effect of the relative humidity on the coefficient of friction. 0,O: ZrB,; A, A: B,C, 0, l : ZrB,+B,C, 0: Zr&+ B,CtSiC. Soiid symbols indicate materials showing Hertzian fracture.

K. Umeda

et al. / Friction

and wear of boride

ceramics

in air and water

65

Fig. 3. Optical micrographs of the sliding damage to the ZrB, block at different relative humidities: (a) 10%; (b) 70%; (c) 90%.

Fig. 4. Optical micrographs of the sliding damage to the B4C block at different relative humidities: (a) 10%; (b) 20%; (c) 30%.

behind the pin slider where the tensile stress reaches its maximum value. The Hertzian fractures occurred on the surfaces of ZrB, for r.h. <70%, on those of B,C for r.h. < 10% and on ZrB, + B,C for r.h. < 20%. No cracking occurred on the sliding surfaces of ZrB, + B,C + Sic composite ceramics, shown in Fig. 6. This fact verifies that toughening of the boride with Sic is effective. The tensile stress behind a spherical rider can be expressed as [8,9]:

where W is the applied load, v is the Poisson ratio, f is the coefficient of friction and u is half of the track width. The critical tensile stress for the Hertzian fracture of each material, then, can be estimated from experimental results using the above equation. The results are shown in Table 3. The estimated critical tensile stress cannot be simply correlated to the bending stress, fracture toughness or hardness.

(T=r

A=

(l;;jw(l+Af) w4+4 8(1- 2v)

(1)

3.2. Friction and wear in de-ionized water Figure 7 shows typical changes in the coefficient of friction during sliding of various ceramics tested. In these tests, every 15 min the sliding speed was increased from 12.5 to 800 mm s-‘. It should be noted that the ZrB, specimen broke, generating a large number of bubbles at the sliding interface, immediately after the

IOOrim Fig. 5. Optical micrographs 30%; (d) 40%.

of the sliding damage

to the ZrBzi-B4C

block at different

relative hhumidities: (a) 10%; (b) 20%;

lOfl,lrm L__“”

.I

Fig. 6. Optical micrographs of the sliding damage to the ZrB,+ B,C+SiC block at different relative humidities: (a) 10%; (b) 70%.

TmM (“h)

Fig. 7. Effect of sliding speed on the coefficient of friction in water. Every 15 min the sliding speed was increased, each step doubling the speed.

K. Umeda et al. / Friction and wear of boride ceramics

Specific wear rate (mm3/N.m)

TABLE 3. Estimated critical tensile stress U: for Hertzian fracture. The superscript * indicates the critical value where the Hertzian fracture initiates Material

a (pm)

f*

0:

ZrB, BJ ZrBz + B,C ZrB, + B,C + Sic

107.7 105 105 107

0.70 0.92 0.85 -

430 577 538 Not fractured

67

in air and water

10-1”

IO-‘3

IO-‘I

“:1’;.:,:,‘.. :;:;i,‘.;; : ‘.

;

(MPa)

H,C

ZrB,+BIC

.y : :y

0’

‘.

IO-”

,0-l,,

1V

,‘:“.:‘,i,‘.:’ Broke”*

y:.::;.

.;(..)y

_.

., _‘: ,:

Fig. 9. Specific wear rate of the tested materials in water. * The specific wear rate of ZrB, is higher than 7.5X10-” mm3 N-’ m-‘. TABLE 4. ICP mass spectrometer

Fig. 8. SEM images of worn surfaces of (a) ZrB,, (b) B4C and (c) ZrB,+B,C+ Sic in water.

test started. The fracture of the specimen occurred as the sliding speed was being increased. This suggested that a severe tribochemical reaction took place, as described later. For other materials tested, no generation

analysis of tested water

Materials

B (mg I-‘)

Zr (mg 1-l)

ZrB, BK ZrB, + B,C ZrB, + B,C + Sic

24.4 0.7 3.6 1.3


of bubbles occurred. For B,C the fluctuation of friction was noticeable at lower sliding velocities, but the value off was as low as 0.02-0.03 at 800 mm s- ‘. The boride composite showed a relatively low value of the coefficient of friction over the range of sliding speeds tested. In particular, the f value of Sic-toughened boride ranged between 0.22 and 0.33 at low sliding speeds, and decreased with increasing sliding speed to as low as 0.03 at the higher speed of 800 mm s-‘. Figure 8 shows SEM images of the worn surfaces of disks after the tests. Grooves noted in the sliding direction are rougher for ZrB, and finer for the SiCtoughened composite. The profiles of worn surfaces of the disks were measured with a surface profilometer at four different positions: north, south, east and west. From measured profilometric areas, the wear volume Vof each specimen was obtained and then the specific wear rates, V/K2 (where L is the total sliding distance) were calculated. Figure 9 shows the specific wear rate of the disks. The wear of B,C was the lowest. In order to understand what kind of tribochemical reaction took place on the sliding surface, the contents of the tested water were examined by means of an inductively coupled plasma method. The results are shown in Table 4. A high boron content was detected in water tested with ZrB,, while less was found in water tested with B,C. Based on this analysis, the following tribochemical reaction is suggested for ZrB,:

68

ZrB, + 10H20 -

K. Umeda et al. / Friction and wear of boride ceramics m air and ~utcr

Zr(OH), + 2H,BO, + 5H,

scheme: (3)

The reason why the composites ZrB,+ B,C and ZrB, + B.,C + Sic undergo less tribological degradation in water may be attributed to the fact that the real sliding contacts take place at B,C or Sic phases, not at ZrB, phases, in these composites, which results in less water degradation.

4. Conclusions The above experimental work may be summarized as follows. The coefficients of friction were about 0.95 for all ceramics at low relative humidity (less than 20%), but decreased with increasing relative humidity. For a high r.h. of about 95%, the friction coefficients ranged from 0.2-0.3, except for that of ZrBz which was about 0.4. Hertzian type fractures were noted on the worn surfaces of ZrB, for r.h. <70%, on those of B,C for r.h. of around lo%, and on those of ZrB,+B,C for r.h. of around 20%. On the other hand, for the ZrB, + B,C + Sic composite such fractures did not occur even at low r.h., even if the friction was high. In de-ionized water, the very severe tribochemical degradation noted for ZrB, was accompanied by hydrogen bubble generation from the sliding surface. The fracture of the test specimen occurred during the test. After the test, boron ions were detected in the test water, leading to the proposal of the following reaction

ZrB, + lOH,O -+

Zr(OH), + 2H,BO, + 5Hz

(41

Fewer boron ions, however, were detected for B,C. ZrB, + B,C and ZrB,+ B,C + Sic. The specific wear rates of these materials were 3.7 x 10 ” mm3 N -’ m ’ for B,C, 3.2 x 1OW” mm3 N-’ m ’ for ZrB, + B,C and 9~10~‘~ mm3 N-’ m-’ for Zr?+B,C+SiC. The coefficients of friction of B,C and ZrB,+B,C+ Sic ranged between 0.22 and 0.33 at low sliding speeds, and decreased with increasing sliding speed to as low as 0.03 at the higher speed of 800 mm s- ‘. The coefficient of friction of ZrB,+B,C ranged between 0.08 and 0.15 over the range of sliding speeds tested. References S. Okumiya, Fine Ceramics ‘87, Fine Ceramics Center, 1987, Nagoya, p. 101. A. Mitsui, K. Mannami, K. Umeda and Y. Enomoto, Jpn. Ceramic Sot., Ann. Meeting, Tokyo, 1988, p. 421. H. Tomizawa and T.E. Fischer, ASLE Trans., 30 (1) (1987) 1. Y. Tsunai and Y. Enomoto, WearofMaterials, ASME, Denver, CO, 1989, p. 369. S. Sasaki, Int. ConjI Wear Mater., ASME, Denver, CO, 1989, p. 409. Y. Enomoto and D. Tabor, Proc. R. Sot. London, Ser. A, 373 (1981) 405-417. Y. Tsuya, Technical Report of Mechanical Engineering Laboratory, Tsukuba, No. 81, 1975. G.M. Hamilton and L.E. Goodman, J. Appl. Mech., 33 (1966) 371. D.R. Gilroy and W. Hirst, J. phys. D, 2 (1969) 1784.