Duan C.G., Karelin V.Y. Abrasive Erosion and Corrosion of Hydraulic machinery

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274

Abrasive Erosion and Corrosion of Hydraulic Machinery

ash or catalyst particles. The critical parts of slurry flow meters suffer

abrasive erosion and cannot indicate slurry flow rate correctly.

For the sound and stable operation of coal liquefaction plants, the

development of materials, which are highly resistant to abrasive erosion, is

indispensable. Several project teams were organized for investigating these

problems and selecting suitable materials for the critical components in the

demonstration plants of coal liquefaction. They reported some results of a

primary investigation in which ceramics and their composites were nominated

for parts of the plants as materials promising high erosion resistance.

Ceramics have a long career as useful materials for industry. They began

to be utilized at about the same time as modern industry was born, and still

now hold an important position together with metallic and organic materials.

Ceramics in the past, familiar to us as tableware, were rather unsuitable for

industrial use because they were brittle in nature and short of reliability in

strength. Their use was, therefore, limited to concrete, brick or glass. With the

recent technical progress in industry, the need has arisen for materials, which

can be used in such severe environments, which even metallic materials can

not bear, and also for those with special characteristics. In response to this

demand, the strength of traditional ceramics has been greatly improved by

selecting and preparing raw materials of high quality and high purity, and also

by applying new manufacturing processes. In addition to the traditional

ceramics of oxides such as Si0

and A1

Ceramics of nitrides (Si

, TIN

etc.),

borides (A1B

TiB

etc.) and carbides (SiC, BC ect.) have been newly

developed. These new ceramics (including the oxides) are endowed not only

with inherent resistance to corrosion and to heat but also with outstanding

electrical, optical and mechanical properties. They are called, therefore, fine

ceramics. Among fine ceramics, those for structural use are called engineering

ceramics (Table 5.16).

Table 5.16 Major engineering ceramics

Nitrides: pSi

, A1N, BN, TiN

Carbides: aSiC, SiC, B

C, WC, TiC

Oxinitrides: Sialon ( Si-Al-O-N )

Oxides: A1

, Zr-Y

, Al

-Zr0

Erosion - Resistant Materials

275

As structural material, engineering ceramics have many advantages:

1> Many are highly resistant to oxidation and other forms of chemical

attack.

2> Some are exceptionally hard and resistant to abrasive erosion.

3> They are usually lighter than metals.

4> They can maintain their mechanical strength at elevated

temperature, and withstand heat.

5> They have small thermal conductivity, small thermal expansion and

high resistance to thermal shock.

Despite their many virtues mentioned above, most ceramics have had their

use greatly restricted in structural applications because of their most glaring

defect, i.e. brittleness. Also, ceramics are often difficult to make in large

complex shapes. Production of the final shape may require expensive

grinding. The machining of some ceramics is difficult, time-consuming, and

costly because of their exceptional hardness.

One of the ways used to cover up the brittleness, in other words to

increase their fracture toughness was to combine a ceramic with another

material. Such a combination is known as a composite material. Ceramic

particles bounded together by metal film (typically consisting of 5 to 15%

volume of the material) is called cermet. Cermet has larger fracture toughness

and is shaped up more easily than bulk ceramics. Another type of composite

is that of a ceramic coating over a substrate of metallic materials which are,

as is well known, easily shaped to any configuration.

5.4.1 Bulk Ceramics

(1) Manufacturing process:

The manufacturing of bulk ceramics begins with the production of pure,

uniform sized, submicron spherical particles of the ceramic material. They are

usually made by a sol-gel process, a solution-precipitation process, or through

vapor phase reaction. Once a ceramic powder is formed, it is mixed with

deliberate additives to improve its strength and other properties, it is then

compacted and shaped at the same time. Dry and wet processing, tape casting,

plastic compaction, cold isostatic pressing, slip casting and injection molding

are the usual processes for the compacting and shaping.

After the compacting and shaping it is densified through sintering at high

temperature (about 1650°C for aluminum oxide, 1700°C for zirconium oxide,

276

Abrasive Erosion and Corrosion of Hydraulic Machinery

2050°C for silicon carbide). During sintering, the ceramic particles coalesce

without actually melting to build up a polycrystalline bulk material. The hot

press (HP) process and the hot isostatic pressing (HIP) process are the

processes in which the compaction and the densification are combined.

(2) Abrasive erosion resistance of bulk ceramics:

D.K. Shetty and his collaborators conducted abrasive erosion tests on a

variety of bulk ceramics in 1983, such as those listed in Table 5.17, where

their physical and mechanical properties are also given. Synthetic slurry

consisting of silica particles (the median size, 10 ~ 15 urn) dispersed in a

petroleum-based oil was heated to a temperature of 220°C. Tests were

conducted by impacting the slurry at a jet velocity of 135 m/sec and a 90°C

degree impingement angle. Test periods were typically 10 minutes in duration.

Erosion rates were calculated from profilmeter traces of the craters produced

on the target surfaces, and expressed as (Av/v) where Av was the volume of

the crater and v was the volume of the target specimen used in the test.

Three different types of abrasive erosion were noted. A group of ceramics

that included a hot pressed B

C, a sintered a-SiC, a hot pressed A1

, a hot

pressed A1N and a glass ceramic exhibited very low erosion rate, smooth

transgranular erosion surface and an approximate inverse correlation of

erosion rates and target hardness as shown in Figure 5.19.

The cemented carbides erosion rates were also plotted in the figure for the

purpose of comparison. The broken lines qualitatively indicate the transition

behavior of cemented carbides, i.e., both hard and brittle grades (low binder

levels) and less hard tougher grades (high binder levels) erode more readily

than do the optimum grades.

A second group of ceramics that included hot pressed grades of SiC and

N4 and a sintered grade of A1

, eroded by an intergranular failure

mechanism, resulted in an order of magnitude increase in erosion rates relative

to the first category.

A third group of ceramics, that is the soda-lime glass without crystalline

grains, was eroded by indentation fracture (lateral cracking) near the center of

craters formed and by microcutting near the periphery. Consistent with its low

hardness (H = 5.2 GPa) and low fracture toughness ( Ki

= 0.75 MPam"

)

this ceramic material exhibited the highest erosion rate among all the

materials tested.

Erosion

Resistant Materials

277

Table 5.17 Properties and erosion rates of target materials

eroded with silica slurry

Material

Cemented

10%Co,4%Cr

Cemented

5.5%

Co,

C(HP)

SiC

(S)

(HP)

AIN(HP)

Glass-

ceramic

SiC (HP)

(HP)

(S)

Soda-lime

glass

Source

(Grade)

Kennametal

(K701)

Kennametal

(K703)

Norton

(Norbide)

Carborundu

Avco

Battelle

Corning

(Pyroceram)

Norton

(NC-203)

Ceradyne

Norton

(NC-132)

3MCo.

(Alsimag614)

PPG Ind.

(Float glass)

Young's

modulus

E (GPa)

533

627

448

406

413

116

440

310

316

Hard-

ness

H (GPa)

16.2

17.0

31.0

26.1

17.7

10.9

6.6

22.7

20.7

15.9

9.4

5.2

Fracture

toughness

1/2

(MPam)

7.8

10.2

2.7

(4.7**)

2.7

(4.7**)

2.9

2.4

4.0

5.3

4.2

4.0

0.75

Erosion

rate *

(Av/v)xl0

1.8+0.7

1.3

0.3+0.15

0.7±0.2

1.4

1.6

210±10

19+2

10+1

5+1

6.5

2200±100

Erosion

charac-

teristics

Transgra-

nular

delaminatio

microflaking

Inter-

granular

Micro-

cutting

indentation

fracture

(HP):

Hot pressed, (S): Sintered.

Test conditions: jet velocity = 135 m/s, 6 = 90, 8 percent silica slurry.

High Kic values reported in the literature.

278 Abrasive Erosion and Corrosion of Hydraulic Machinery

10"

10"'

S.L. Glass

10"'

10"

8% Silica Slurry

V = 135 m/s

6 = 90

/cemented Carbiides

Al N (PH)

k-703

Silica

*AI

(HP)

a-SiC (s)

C (HP)

10 20 30

Target Hardness, H(GPa)

100

Figure 5.19 Erosion rate (Av/v) - hardness (H) correlation

for ceramic targets that eroded by the delamination

of micro-flaking mechanism

The above test results may be summarized as follows. The bulk ceramics

of polycrystalline structure erode by either of two mechanisms, transgranular

or intergranular failure. The latter showed significantly higher erosion rates.

Erosion - Resistant Materials

279

However, the fracture toughness value of the ceramics eroded by

intergranular failure was in fact greater than the toughness value of the

ceramics that eroded by transgranular failure. Thus bulk fracture toughness is

not a good measure of a ceramic's resistance to erosion. On the other hand,

hardness is a good measure of erosion resistance once the fracture category is

decided.

Another problem in the erosion of bulk ceramics is that different grades of

the same generic material can be eroded by the two different mechanisms

showing an order of magnitude difference in erosion rates.

5.4.2 Cemented Carbides

(1) Manufacturing process

Cermet consists essentially of

the

particles of brittle but hard carbides, oxides

or nitrides, contained in a ductile binder phase. In a broad sense, the carbide

element may be one of those, which belong to the group IV

, V

, VI

in the

periodic table, and the binder metal is the iron group, namely Fe, Ni and Co.

But at present, only two combinations, i.e., WC-Co and WC-TiC(TaC, Nbc)-

Co are in practical use. These are usually called cemented carbides.

As shown in Figure 5.20, the standard manufacturing process of cemented

carbides has already been established. Powdered tungsten carbide, cobalt

metal and additions are mixed together, pressed to form a block, and then

sintered at an elevated temperature. HIP (hot isostatic pressing) may be used

instead of sintering. In the early stage of sintering, a liquid solution is created

from the Co and WC at a certain temperature lower than the melting point of

WC.

Sintering, then, proceeds rapidly as the solution enfolds the WC

particles. In the cooling process following the sintering, the tungsten and

carbon elements in the solution precipitate on the surface of the WC as

crystal, and cobalt increases its concentration in the solution. The nearly pure

cobalt metal solidifies itself in the space between the WC particles and makes

a strong bonding there. Formation of the liquid solution, to adequately cover

the particle's surface, is indispensable for a strong bonding.

(2) Properties of cemented carbides:

It has been already made clear that the mechanical properties of cemented

carbides depend on the composition and size of

the

carbide, and most strongly

on WC/Co ratio. Generally a larger WC/Co ratio and smaller particle size

brings about a lower toughness but a higher hardness. In an ideal condition

280

Abrasive Erosion and Corrosion of Hydraulic Machinery

WC crystals develop themselves in a square pillar shape because they have a

hexagonal lattice. Crystals in this shape increase the toughness of the

material.

WC powder]

Co powder

> >

Mixing

Pressing

Pre - Sintering

other powderj

Machining ^T™

Figure 5.20 Manufacturing process of cemented carbides.

(3) Erosion resistance:

The chemical composition and the mechanical property of cemented carbide

specified by JIS (Japanese Industrial Standard) are shown in Table 5.18.

Erosion tests were conducted on these materials using a rotary type testing

apparatus which is similar to the one shown in Figure 5.17 of the preceding

paragraph, under the following conditions; abrasive particles, SiC (125 urn, #

120);

particle concentration in slurry (water), 10 wt%; the maximum

peripheral velocity at the tip of propeller, 10.3 m/sec; duration of test, 44

hours.

Test results obtained are shown in Figure 5.21 as relationships between

the hardness of material and the relative extent of damage, taking stainless

steel (SUS 304) as the standard. Most materials fell on the line which

indicated that the higher the hardness is, the smaller the extent of the damage.

TF2,

which is of the highest hardness among the materials tested shows the

smallest extent of damage. Several materials, however, did not obey the

hardness vs. damage relation. This may be partly attributed to the fact that

those were mainly A - and p-type and they contained TiC and TaC. But, some

materials without TiC or TaC did not obey the relation, and trials to relate the

erosion resistance with other properties of the material such as WC/Co ratio

or particle size also failed. Therefore, we cannot but conclude that it is

impossible to relate the abrasive erosion resistance of cemented carbides with

any single property of them. This conclusion is supported by another test

results shown in Figure 5.19. Of course, we have to keep it in mind that as a

general rule, erosion resistance depends decisively on the test conditions.

Erosion

Resistant Materials

281

Table 5.18 Chemical composition and mechanical properties

of cemented carbides

speci-

fication

M10

M40

P10

P30

K20

Ultra

fine

Material name

MV1

TV1

MT3

TV3

MV6

TV6

MM1

TM1

MM4

TM4

MP1

TP1

MP3

TP3

MK2

MF1

MF2

MF3

TF1

TF2

MR1

MR2

TR1

DTi05

D10

GTi20

D30

GTi40S

D60

UTilOT

TU40

UTi40T

TU40

STilOT

TX10

STi30

Tx30

HTi20

UF20

UF30

HF25

GTI30C

GTI50C

MS 18

Chemical

composition

(%)

5.5

5.7

Co Ni

7 7

11 11

18 2

TiC

TaC

—

...

17.7

6.5

9.5

30.6

10.5

—

2.0

...

rest

Mechanical property

Hardness

HRa

91.0

90.5

89.0

88.0

83.0

84.5

92.0

92.5

89.0

91.5

92.0

90.5

91.1

91.5

90.0

91.0

93.0

84.0

80.0

86.0

1613

1483

1272

1158

788

831

1545

1693

1263

1693

1532

1651

1495

1583

1570

1382

1651

1557

1892

867

744

1037

Deflective

strength

(kg/mm

)

200

280

300

310

320

150

230

240

160

150

200

180

220

250

300

250

200

230

250

180

Specific

gravity

14.9

14.4

14.1

13.5

13.0

11.1

13.0

13.5

12.4

10.4

10.5

13.0

12.7

14.9

14.2

13.8

14.0

14.3

14.9

13.9

13.5

13.6

282 Abrasive Erosion and Corrosion of Hydraulic Machinery

J—•—i—i—i

i i • —i—i—i—i * i ■ i i i i—

SOO

1.000 1500 2.000

Target Hardness (Hv)

Figure 5.21 Target hardness vs relative erosion rate correlation

for cemented carbides

5.4.3 Coatings

(1) Manufacturing process

Special techniques are required for the preparation of ceramic films and

coatings. Figure 5.22 shows a classification of the techniques: In the chemical

vapor deposition (CVD) method a gaseous or volatilized compound is

transported by a pressure difference or by a carrier gas to the substrate

surface which is maintained at a much higher temperature than the vapor

source, but at a considerably lower temperature than the melting temperature

of deposit compound. The deposition processes take place on the substrate

surface in the form of decomposition or chemical reduction obeying the laws

of thermodynamics and reaction kinetics. The following is an example of

CVD reaction

1000°C

3TiCl

(g) + C

(g) + 2H

(g) ► 3TiC (S) + 12Hcl (g)

onFe

Erosion - Resistant Materials

283

Ceramic coating method

Chemical vapor depositon (CVD)

Physical vapor depositon (PVD)

Thermal spraying

Reduction CVD

Decomposition CVD

Plazma enhanced CVD

Evaporation

Sputtering

Ion plating

Plazma spraying

Flame spraying

Detonation

gun

Figure 5.22 Ceramic coating methods

In physical vapor deposition (PVD) techniques, the material needing

coating is placed in a high vacuum chamber (typically 10"

torr) opposite the

substrate and evaporated by heating or electron beam. Upon evaporation,

atoms of coating material are ejected from the solid material, and impinge and

condense on the substrate surface (Evaporation method). Or, the ejection of

the source material is accomplished by the bombardment of the surface with

gas ions accelerated by a high voltage (Sputtering method). When a high

voltage is applied between the substrate (-) and the source material (+) in a

vacuum but with some leaked gas (Ar, N

) of 10"

~ 10"

torr, a plasma state

is established and the particles of atomic dimension from the source metal

react with the gas to form a compound on the substrate surface (Ion plating

method). An example of

ion

plating is

6.0xlO"

2Ti + N

2Tj N 773k

The thermal spraying process is a technique that combines solid particle

melting, quenching and consolidation all together. In plasma spraying, a

plasma jet is created by electric heating an inert gas arc in a water-cooled