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Crystalline boron and its abrasive properties
477
Journal of the Less-Common Metals, 67 (1979) 477 - 484 0 Ehevier Sequoia S.A., Lausanne -Printed in the Netherlands
CRYSTALLINE BORON AND ITS ABRASIVE PROPERTIES*
E. A. KNYSHEV, V. A. KOBYAKOV, K. V. TKATCHEV, E. I. MASHKOVSKII and G. G. KARYUK
K. P. TSOMAYA,
Ural Institute of Chemistry, Suerdlovsk fU.SY3.R.)
Summary Samples of crystalline p rhombohedral boron were prepared. It was found that boron powders with grain sizes of 1 -28 E.tmcan be obtained by mechanical dispersion followed by grading. Abrasive pastes based on these powders were prepared and their ability to grind and polish silicon hard steels and non-ferrous metals was tested. It was established that the pastes under investigation compared well with traditional carbide-based pastes. The degree of purity of the surfaces treated with these pastes was 1 - 2 times higher than the degree of purity of the surfaces obtained with standard pastes. It was found that doping boron with some elements, e.g. with 6 wt.% C, increased its microhardness up to 4500 5000 kg mmW2and improved the crushing strength of the grains.
The intensive development of new industries in which high velocities, high temperatures, high pressures and very reactive substances are used requires the development of new materials. The refractory metals of groups IV-VI, their alloys and their compounds (e.g. with boron) appear to be most promising because of their useful properties and because of technical and economic factors. Borides and boron are being increasingly used for heat-resistant alloys and automatic devices in rocket building, in the nuclear energy industry, in electronics and in the abrasive industry. Crystalline boron when used as an abradant helps to solve some of the problems that arise with non-tungsten abradants and other abrasive finishing methods and enables a smoother surface to be achieved. The combination of a high microhardness, a high cleavage grain strength and the brittleness of crystalline boron make it competitive with the well-known abradants such as artificial diamonds, B4C, cubic BN etc. In the present work the abrasive properties of p rhombohedral crystalline boron and of crystalline boron doped with 4 - 6 at.% C were studied,
*Paper presented at the 6th International Symposium on Boron and Borides, Varna, Bulgaria, October 9 - 12,1978.
478
Boron ingots of mass 150 - 2000 g were prepared in electron beam and induction furnaces by melting amorphous boron. The phases present were identi~ed by X-ray analysis. The majority of the research was devoted to the grinding and grading of the polishing powders and micropowders, in spite of some difficulties due to the strength of boron and its high microhardness. The milling of large parts and ingots of boron was carried out with the help of a jaw breaker with a hard alloy arm. By milling four times, we obtained milled material of particle size less than 0.1 pm. Particles were usually needle or plate shaped with some cracks and defect grains. The grain shape was standardized in an MLA-3 impactor and in a vertical layer apparatus. The rough powders (of particle size greater than 2 pm) were graded by decant&ion. An aqueous solution of gelatin (l/l g 1-l) and soda ash (0.22 g 1-l) was used as the suspension. About 0.5 - 1.0 kg of the powder was placed in 11 of the solution and was thoroughly agitated; this mixture was then allowed to settle for 12 - 14 h. Boron particles of different sizes settle at different rates and were therefore distributed at different heights in the dispersion fluid. Separation of the different fractions was achieved by decanting the suspension layers in series. In order to grade the fine powder fractions (of particle size less than 1.4 ,um) even more, a small glass centrifuge was used. The success of this grading method was verified visually with a microscope and also by a Coulter counter. Electron microscope ~vestiga~ons showed that the grading scheme we used allowed distinct separation of the different powder fractions. Simultaneously it was found that the number of boron grains increased as the grain size decreased; this was useful in grinding, The flow diagram for processing the crystalline boron micropowders is shown in Fig. 1. The effects that the main abrasive properties, i.e. the roughness of the treated surface and the grinding depth, of the boron micropowders have on silicon, steel, brass and diamond and their dependence on the pressure exerted per unit area, the grinding time, the boron micropowder concentration in the paste, the powder size and the carbon content in boron were investigated. Table 1 contains some data which are characteristic of crystalline boron; similar data for other abradants are shown for comparison. As can be seen, the properties of crystalline boron are very like those of traditional abradants and in some cases they are even better. The principal parameter of surface quality estimation, which is used both in grinding investigations and in studying abradants, is roughness, i.e. the amount of unevenness, which determines the surface purity. The next most impost parameter for the estimation of abrasive treatment is the amount of material ground per unit time (the grinding rate). Boron micropowders were used as pastes with a grease base [l]. On a GK-1409 grinding machine ~ron~on~ing paste was used to grind ballbearing steel, brass and silicon plates 5 mm thick. The paste contained 30%
479 Crystalline
boron
I Jaw breaker and hammer breaker milling I Milling by VA-100
or AVSP-100
I Static grading with powders 60 - 40,40 - 28, 28 - 20, 20 - 14 @rn and suspension; 14 pm grains separated
I i
1’ Washing by water and aqueous ethyl alcohol powders 60 - 40,40 - 28, 28 - 20,20 - 14 j.Lm ----l---
I
Vacuum drying
Centrifugation
I 14-10
of suspension by centrifuge less than 14 pm separated
I ‘10-7
I 7-s
I 5-3
Itm
3-o
pm Izm / V Washing by water and aqueous ethyl alcohol
pm
CK-4; grains
pm Centrifugation by supercentrifuge c-100
I
1 Packing
4 Washing by water and aqueous ethyl alcohol
Vacuum drying I Packing Fig. 1, The process flow diagram for the production
of crystalline
boron micropowders.
boron powder of grain size 20 - 14 pm, 20% industrial oil, 30% oleic acid and 20% stearine. A profilograph (with an indenter radius of not more than 2 pm) and an MII-4 mi~roin~rferome~r were used to estimate the quality of the treated surface. The grinding rate was measured by the change in the height of the plate (an average of several measurements was taken); this gave the grinding depth. Pressure per unit area is one of the main factors which determines whether the grinding is successful either from an efficiency or from a surface quality point of view. The influence that pressure has on the grinding depth (which indicates how efficient the process is) was studied using silicon samples. The total grinding depth in 5 min was measured for various pressures in the range 0.125 - 0.500 kgfcme2. The results obtained are shown in Fig. 2, curve 1. A marked increase in the grinding depth occurred as the pressure
sour
results.
Crystalline boron Crystalline boron doped wt.% c!
B4C
with 4 - 6
Hexagonal Cubic Rhombohedral Rhombohedral Rhombohedrai
Cubic -
corundum
and other
Diamond ACB BlectroIyticaiIy produced Natural corundum Sic (green) BN
of boron Type of structure
properties
Material
Some physicomechsnical
TABLE 1
3.48 - 3.54 3.9 3.2 3.5 2.52 2.35 -
Density (g cmm3)
abradants
660 326 540 720 520a 854a
12 770 445 650 770 150 690 891a
16 900 800 750 925
20
916 116B8
-
Cleavage grain strength (g grain-l ) for three grain sizes in pm 8000 1800 2000 3000 8000 4930 3800 4500
- 4300a - 5200a
- 10000 - 2400 - 2200 - 3500 - 10000
Microhardness (kg mm-l)
1,394 l-3,5 4 1,294 1,3 233 5,6
Reference
481
kk
z
“b” To
Q%5mm o;e
kg/ rn’
Fig. 2. The dependence of the sample wear attrition on the pressure and on the duration of treatment.
I
IO
(4
Paste concentration (9%)
(b)
20
20
40
50
60 c-x
Paste concentration (%I
Fig. 3. (a) The dependence of the sample wear attrition on the paste concentration; (b) the dependence of the surface roughness on the paste concentration.
was increased from 0.125 to 0.375 kgf cm-‘; the increase in the grinding depth then slowed down until eventually above 0.500 kgf cmm2 there was no further increase at all. It is well known that grinding slows down when the abradant grains crack [ 2,3]. We showed that in our samples the optimal pressure was 0.4 kgf cme2. The same type of dependence and the same optimal pressure have been reported for other abradants [ 1,3]. The roughness of the surface treated by the boron micropowder paste corresponds to a grade 9 surface. The grinding depth changes with time. It is maximal during the first few minutes of grinding [ 31. In the experiments with silicon samples the
the grinding duration was 5 - 15 min, the pressure was 250 gf cms2 and the boron concentration in the paste was 30%. The results are shown in Fig. 2, curve 2. During the first 5 min the depth ground was 34 pm, during the next 5 min it decreased to 47% of the initial value and finally it fell to 70% of the initial value. The surface roughness was unchanged and corresponded to Ra = 0.32 pm or grade 9. The influence of the boron micropowder content in the paste on the grinding depth and on the roughness was investigated using silicon samples for a pressure of 250 gf cmm2 and a grain size in the paste of 20 - 14 pm; the grinding duration was 5 min and the rotational velocity was 46 min-*. 1.5 g of the paste was used for one series. The dependence of the boron content in the paste on the roughness and on the wear attrition is very similar to that found for other abrasive pastes [l, 21. It can be seen from Fig. 3 that, as the boron content was increased from 10 to 30%, the depth ground increased by 1.3 times. However, when the boron content was increased to over 50%, the output decreased. There was also a simultaneous worsening in the surface quality. The roughness of the surface corresponded to grade 9 for a boron content in the paste of less than 40%, but for a boron content of 40 - 80% the surface roughness was only grade 8. The changes in the surface roughness and its dependence on the boron content in the paste is shown in Fig. 3(b). By analogy with the investigations described in ref. 3 we can assume that, as the abradant (the boron) content was increased, the boron quickly saturated with slurry, thereby scratching the treated surface. A comparison of Fig. 3(a) with Fig. 3(b) shows that when grinding silicon the optimal limit of boron content in the paste is 30 - 50%. A more accurate definition of the optimal content can be made by taking into account the surface quality required and the cost allowed. If a high surface quality is desired, this would necessitate expensive and complicated devices; obviously it might be better to tolerate a less efficient grinding (finishing) process. We investigated the dependence of the grinding depth and the surface roughness on the grain size of the boron in the paste using silicon, brass and ball-bearing steel plates; the boron content in the paste was 30% and the grain sizes were 5 - 7, 7 - 10, 14 - 20 and 20 - 28 pm. The pressure was 500 gf cmm2, the duration was 5 min and the rotational velocity was 46 min-l. The results obtained are plotted in Fig. 4(a) and Fig. 4(b). The grinding depth increased as the grain size increased. This effect was most marked when grinding brass, it was less for silicon and it was least of all for steel. Tbe differences between the samples decreased as the grain size decreased. The largest grinding depth, 60 grn in 5 min, was achieved with brass and a grain size of 20 - 14 pm (Fig. 4(a)). Similar results were obtained for the surface roughness also. The best result was obtained for small boron grains. The smaller the grain size, the smaller is the surface roughness Ra (Fig. 4(b)). The differences between the samples decreased as the grain size decreased; however, the shape of the
483
%&I%5
(a)
Grain size of paste (rml
fbf
=Ym %
30
‘95
75 %=YA
Grain size of paste (pm)
Fig. 4. (a) The relationship between the sample wear attrition and the boron grain size for brass (curve l), silicon (curve 2) and ball-bearing steel (curve 3); (b) the relationship between the surface roughness and the boron grain size for silicon (curve l), ballbearing steel (curve 2) and brass (curve 3).
curve for brass is slightly different. A roughness Ra = 0.1 ~.trnwas obtained for steel using a boron gram size of 7 - 5 pm. This roughness corresponds to grade 10. Analogous dependences have been shown for other abradants [l - 31. These effects can be explained by the fact that the cutting ability of an abradant is greater when more grains are situated on the cutting tool surface (the number of grams increases as the grain size decreases). We investigated the influence of the carbon content in the boron on the microhardness (Table 1) of the boron; carbon was added to improve the grinding results. The same experimental conditions as mentioned above were employed. For comparison, experiments using the abradant powders of artificial diamond ACP (State Standard 9207-70)) B4C and ZrC were also carried out under the same experimental conditions. The results (averaged) are shown in Table 2. The grinding depth of steel increased with increase in the carbon content; the surface roughness remained unchanged. The grinding depth of brass increased only for a boron paste containing 6% C; the surface roughness remained unchanged. A comparison of boron with other abradants (Table 2) shows that boron paste achieved a lower surface roughness and a smaller grinding depth for brass than artificial diamond or B,C did. The experimental data obtained on boron micropowders for the abrasive treatment of different materials allows us to conclude that crystalline boron is a highly effective abradant.
484 TABLE 2 Test results of boron of grain size 28 - 20 pm in a paste (30% B) and the dependence on the carbon content in comparison with other abradants Material
Ball-bearing
steel
Grinding
Ra
depth B+2%C B+4%C B+6%C Artificial B& ZrC
diamond
ACP
15 17 28 24 32 10
(pm) (pm) 0.18 0.14 0.21 0.33 0.57 0.18
Brass Roughness grade 9b 10a :b 8a 9b
Grinding
Ra
Roughness
depth
(pm)
grade
0.,40 0.,40 0.40 0.5 0.6 0.16
8b 8b 8b 8a 8a 9b
60 56 76 16 37 15
(pm)
References 1 P. A. Shulman et al., Roughness of Diamond Treated Surfaces, Tekhnika, Kiev, 1972. 2 G. B. Lurie, Grinding of Metals, Mashinostroenie, Moscow, 1969, pp. 98 - 99. 3 Diamond Abrasive Treatment and Finishing Operations, Moscow, 1969, pp, 3 - 9, 10 - 16,174 - 185. 4 V. F. Mgeladze and V. V. Karain, Ab~ives, (2) (1971) 3. 5 G. V. Samsonov, L. Ya. Markovski, A. F. Zhygach and M. G. Valyashko, Boron, its Compounds and Alloys, Akademii Nauk Ukrainskoi S.S.R., Kiev, 1960. 6 G. V. Samsonov, Powder Metall., (8) (1973) 58.
Journal of the Less-Common Metals, 67 (1979) 477 - 484 0 Ehevier Sequoia S.A., Lausanne -Printed in the Netherlands
CRYSTALLINE BORON AND ITS ABRASIVE PROPERTIES*
E. A. KNYSHEV, V. A. KOBYAKOV, K. V. TKATCHEV, E. I. MASHKOVSKII and G. G. KARYUK
K. P. TSOMAYA,
Ural Institute of Chemistry, Suerdlovsk fU.SY3.R.)
Summary Samples of crystalline p rhombohedral boron were prepared. It was found that boron powders with grain sizes of 1 -28 E.tmcan be obtained by mechanical dispersion followed by grading. Abrasive pastes based on these powders were prepared and their ability to grind and polish silicon hard steels and non-ferrous metals was tested. It was established that the pastes under investigation compared well with traditional carbide-based pastes. The degree of purity of the surfaces treated with these pastes was 1 - 2 times higher than the degree of purity of the surfaces obtained with standard pastes. It was found that doping boron with some elements, e.g. with 6 wt.% C, increased its microhardness up to 4500 5000 kg mmW2and improved the crushing strength of the grains.
The intensive development of new industries in which high velocities, high temperatures, high pressures and very reactive substances are used requires the development of new materials. The refractory metals of groups IV-VI, their alloys and their compounds (e.g. with boron) appear to be most promising because of their useful properties and because of technical and economic factors. Borides and boron are being increasingly used for heat-resistant alloys and automatic devices in rocket building, in the nuclear energy industry, in electronics and in the abrasive industry. Crystalline boron when used as an abradant helps to solve some of the problems that arise with non-tungsten abradants and other abrasive finishing methods and enables a smoother surface to be achieved. The combination of a high microhardness, a high cleavage grain strength and the brittleness of crystalline boron make it competitive with the well-known abradants such as artificial diamonds, B4C, cubic BN etc. In the present work the abrasive properties of p rhombohedral crystalline boron and of crystalline boron doped with 4 - 6 at.% C were studied,
*Paper presented at the 6th International Symposium on Boron and Borides, Varna, Bulgaria, October 9 - 12,1978.
478
Boron ingots of mass 150 - 2000 g were prepared in electron beam and induction furnaces by melting amorphous boron. The phases present were identi~ed by X-ray analysis. The majority of the research was devoted to the grinding and grading of the polishing powders and micropowders, in spite of some difficulties due to the strength of boron and its high microhardness. The milling of large parts and ingots of boron was carried out with the help of a jaw breaker with a hard alloy arm. By milling four times, we obtained milled material of particle size less than 0.1 pm. Particles were usually needle or plate shaped with some cracks and defect grains. The grain shape was standardized in an MLA-3 impactor and in a vertical layer apparatus. The rough powders (of particle size greater than 2 pm) were graded by decant&ion. An aqueous solution of gelatin (l/l g 1-l) and soda ash (0.22 g 1-l) was used as the suspension. About 0.5 - 1.0 kg of the powder was placed in 11 of the solution and was thoroughly agitated; this mixture was then allowed to settle for 12 - 14 h. Boron particles of different sizes settle at different rates and were therefore distributed at different heights in the dispersion fluid. Separation of the different fractions was achieved by decanting the suspension layers in series. In order to grade the fine powder fractions (of particle size less than 1.4 ,um) even more, a small glass centrifuge was used. The success of this grading method was verified visually with a microscope and also by a Coulter counter. Electron microscope ~vestiga~ons showed that the grading scheme we used allowed distinct separation of the different powder fractions. Simultaneously it was found that the number of boron grains increased as the grain size decreased; this was useful in grinding, The flow diagram for processing the crystalline boron micropowders is shown in Fig. 1. The effects that the main abrasive properties, i.e. the roughness of the treated surface and the grinding depth, of the boron micropowders have on silicon, steel, brass and diamond and their dependence on the pressure exerted per unit area, the grinding time, the boron micropowder concentration in the paste, the powder size and the carbon content in boron were investigated. Table 1 contains some data which are characteristic of crystalline boron; similar data for other abradants are shown for comparison. As can be seen, the properties of crystalline boron are very like those of traditional abradants and in some cases they are even better. The principal parameter of surface quality estimation, which is used both in grinding investigations and in studying abradants, is roughness, i.e. the amount of unevenness, which determines the surface purity. The next most impost parameter for the estimation of abrasive treatment is the amount of material ground per unit time (the grinding rate). Boron micropowders were used as pastes with a grease base [l]. On a GK-1409 grinding machine ~ron~on~ing paste was used to grind ballbearing steel, brass and silicon plates 5 mm thick. The paste contained 30%
479 Crystalline
boron
I Jaw breaker and hammer breaker milling I Milling by VA-100
or AVSP-100
I Static grading with powders 60 - 40,40 - 28, 28 - 20, 20 - 14 @rn and suspension; 14 pm grains separated
I i
1’ Washing by water and aqueous ethyl alcohol powders 60 - 40,40 - 28, 28 - 20,20 - 14 j.Lm ----l---
I
Vacuum drying
Centrifugation
I 14-10
of suspension by centrifuge less than 14 pm separated
I ‘10-7
I 7-s
I 5-3
Itm
3-o
pm Izm / V Washing by water and aqueous ethyl alcohol
pm
CK-4; grains
pm Centrifugation by supercentrifuge c-100
I
1 Packing
4 Washing by water and aqueous ethyl alcohol
Vacuum drying I Packing Fig. 1, The process flow diagram for the production
of crystalline
boron micropowders.
boron powder of grain size 20 - 14 pm, 20% industrial oil, 30% oleic acid and 20% stearine. A profilograph (with an indenter radius of not more than 2 pm) and an MII-4 mi~roin~rferome~r were used to estimate the quality of the treated surface. The grinding rate was measured by the change in the height of the plate (an average of several measurements was taken); this gave the grinding depth. Pressure per unit area is one of the main factors which determines whether the grinding is successful either from an efficiency or from a surface quality point of view. The influence that pressure has on the grinding depth (which indicates how efficient the process is) was studied using silicon samples. The total grinding depth in 5 min was measured for various pressures in the range 0.125 - 0.500 kgfcme2. The results obtained are shown in Fig. 2, curve 1. A marked increase in the grinding depth occurred as the pressure
sour
results.
Crystalline boron Crystalline boron doped wt.% c!
B4C
with 4 - 6
Hexagonal Cubic Rhombohedral Rhombohedral Rhombohedrai
Cubic -
corundum
and other
Diamond ACB BlectroIyticaiIy produced Natural corundum Sic (green) BN
of boron Type of structure
properties
Material
Some physicomechsnical
TABLE 1
3.48 - 3.54 3.9 3.2 3.5 2.52 2.35 -
Density (g cmm3)
abradants
660 326 540 720 520a 854a
12 770 445 650 770 150 690 891a
16 900 800 750 925
20
916 116B8
-
Cleavage grain strength (g grain-l ) for three grain sizes in pm 8000 1800 2000 3000 8000 4930 3800 4500
- 4300a - 5200a
- 10000 - 2400 - 2200 - 3500 - 10000
Microhardness (kg mm-l)
1,394 l-3,5 4 1,294 1,3 233 5,6
Reference
481
kk
z
“b” To
Q%5mm o;e
kg/ rn’
Fig. 2. The dependence of the sample wear attrition on the pressure and on the duration of treatment.
I
IO
(4
Paste concentration (9%)
(b)
20
20
40
50
60 c-x
Paste concentration (%I
Fig. 3. (a) The dependence of the sample wear attrition on the paste concentration; (b) the dependence of the surface roughness on the paste concentration.
was increased from 0.125 to 0.375 kgf cm-‘; the increase in the grinding depth then slowed down until eventually above 0.500 kgf cmm2 there was no further increase at all. It is well known that grinding slows down when the abradant grains crack [ 2,3]. We showed that in our samples the optimal pressure was 0.4 kgf cme2. The same type of dependence and the same optimal pressure have been reported for other abradants [ 1,3]. The roughness of the surface treated by the boron micropowder paste corresponds to a grade 9 surface. The grinding depth changes with time. It is maximal during the first few minutes of grinding [ 31. In the experiments with silicon samples the
the grinding duration was 5 - 15 min, the pressure was 250 gf cms2 and the boron concentration in the paste was 30%. The results are shown in Fig. 2, curve 2. During the first 5 min the depth ground was 34 pm, during the next 5 min it decreased to 47% of the initial value and finally it fell to 70% of the initial value. The surface roughness was unchanged and corresponded to Ra = 0.32 pm or grade 9. The influence of the boron micropowder content in the paste on the grinding depth and on the roughness was investigated using silicon samples for a pressure of 250 gf cmm2 and a grain size in the paste of 20 - 14 pm; the grinding duration was 5 min and the rotational velocity was 46 min-*. 1.5 g of the paste was used for one series. The dependence of the boron content in the paste on the roughness and on the wear attrition is very similar to that found for other abrasive pastes [l, 21. It can be seen from Fig. 3 that, as the boron content was increased from 10 to 30%, the depth ground increased by 1.3 times. However, when the boron content was increased to over 50%, the output decreased. There was also a simultaneous worsening in the surface quality. The roughness of the surface corresponded to grade 9 for a boron content in the paste of less than 40%, but for a boron content of 40 - 80% the surface roughness was only grade 8. The changes in the surface roughness and its dependence on the boron content in the paste is shown in Fig. 3(b). By analogy with the investigations described in ref. 3 we can assume that, as the abradant (the boron) content was increased, the boron quickly saturated with slurry, thereby scratching the treated surface. A comparison of Fig. 3(a) with Fig. 3(b) shows that when grinding silicon the optimal limit of boron content in the paste is 30 - 50%. A more accurate definition of the optimal content can be made by taking into account the surface quality required and the cost allowed. If a high surface quality is desired, this would necessitate expensive and complicated devices; obviously it might be better to tolerate a less efficient grinding (finishing) process. We investigated the dependence of the grinding depth and the surface roughness on the grain size of the boron in the paste using silicon, brass and ball-bearing steel plates; the boron content in the paste was 30% and the grain sizes were 5 - 7, 7 - 10, 14 - 20 and 20 - 28 pm. The pressure was 500 gf cmm2, the duration was 5 min and the rotational velocity was 46 min-l. The results obtained are plotted in Fig. 4(a) and Fig. 4(b). The grinding depth increased as the grain size increased. This effect was most marked when grinding brass, it was less for silicon and it was least of all for steel. Tbe differences between the samples decreased as the grain size decreased. The largest grinding depth, 60 grn in 5 min, was achieved with brass and a grain size of 20 - 14 pm (Fig. 4(a)). Similar results were obtained for the surface roughness also. The best result was obtained for small boron grains. The smaller the grain size, the smaller is the surface roughness Ra (Fig. 4(b)). The differences between the samples decreased as the grain size decreased; however, the shape of the
483
%&I%5
(a)
Grain size of paste (rml
fbf
=Ym %
30
‘95
75 %=YA
Grain size of paste (pm)
Fig. 4. (a) The relationship between the sample wear attrition and the boron grain size for brass (curve l), silicon (curve 2) and ball-bearing steel (curve 3); (b) the relationship between the surface roughness and the boron grain size for silicon (curve l), ballbearing steel (curve 2) and brass (curve 3).
curve for brass is slightly different. A roughness Ra = 0.1 ~.trnwas obtained for steel using a boron gram size of 7 - 5 pm. This roughness corresponds to grade 10. Analogous dependences have been shown for other abradants [l - 31. These effects can be explained by the fact that the cutting ability of an abradant is greater when more grains are situated on the cutting tool surface (the number of grams increases as the grain size decreases). We investigated the influence of the carbon content in the boron on the microhardness (Table 1) of the boron; carbon was added to improve the grinding results. The same experimental conditions as mentioned above were employed. For comparison, experiments using the abradant powders of artificial diamond ACP (State Standard 9207-70)) B4C and ZrC were also carried out under the same experimental conditions. The results (averaged) are shown in Table 2. The grinding depth of steel increased with increase in the carbon content; the surface roughness remained unchanged. The grinding depth of brass increased only for a boron paste containing 6% C; the surface roughness remained unchanged. A comparison of boron with other abradants (Table 2) shows that boron paste achieved a lower surface roughness and a smaller grinding depth for brass than artificial diamond or B,C did. The experimental data obtained on boron micropowders for the abrasive treatment of different materials allows us to conclude that crystalline boron is a highly effective abradant.
484 TABLE 2 Test results of boron of grain size 28 - 20 pm in a paste (30% B) and the dependence on the carbon content in comparison with other abradants Material
Ball-bearing
steel
Grinding
Ra
depth B+2%C B+4%C B+6%C Artificial B& ZrC
diamond
ACP
15 17 28 24 32 10
(pm) (pm) 0.18 0.14 0.21 0.33 0.57 0.18
Brass Roughness grade 9b 10a :b 8a 9b
Grinding
Ra
Roughness
depth
(pm)
grade
0.,40 0.,40 0.40 0.5 0.6 0.16
8b 8b 8b 8a 8a 9b
60 56 76 16 37 15
(pm)
References 1 P. A. Shulman et al., Roughness of Diamond Treated Surfaces, Tekhnika, Kiev, 1972. 2 G. B. Lurie, Grinding of Metals, Mashinostroenie, Moscow, 1969, pp. 98 - 99. 3 Diamond Abrasive Treatment and Finishing Operations, Moscow, 1969, pp, 3 - 9, 10 - 16,174 - 185. 4 V. F. Mgeladze and V. V. Karain, Ab~ives, (2) (1971) 3. 5 G. V. Samsonov, L. Ya. Markovski, A. F. Zhygach and M. G. Valyashko, Boron, its Compounds and Alloys, Akademii Nauk Ukrainskoi S.S.R., Kiev, 1960. 6 G. V. Samsonov, Powder Metall., (8) (1973) 58.