Tribological properties of boride based thermal diffusion coatings

Abstract

Engineering components, e.g. tubing systems for the down-hole applications in the oil and gas industry (in particular, sucker rod pumps, progressing cavity pumps and some other components of the artificial lifting systems), as well as numerous valves and seats, bearings, gears and plungers, require protection against friction and sliding abrasion service conditions. The hard boride based coatings on steels and alloys obtained through the thermal diffusion process have a high potential for these severe application conditions over many other types of coatings as they can be obtained on the entire working surfaces of large size and complex shape products. Intensive tribological studies of the iron boride based coatings on carbon steel obtained at Endurance Technologies Inc. have been conducted using the Cameron-Plint testing unit (reciprocating sliding of the metallic rod under the load over a flat surface of the coated samples). The friction wear loss, friction coefficient and structural changes of the coatings have been studied in dry and lubricating (water-oil) friction conditions, which simulate actual application conditions. It was demonstrated that the obtained boride coatings have the friction loss significantly smaller than untreated steel (e.g. y10-30 times in the dry conditions and at least 5 times in the lubricating conditions) with no peeling and flaking-off. The friction coefficients of the boride coatings are steady over the test duration. The influence of the thickness on the boride coatings performance is demonstrated. The encouraging results are explained by the specific coating structure of the hard coating obtained through the thermal diffusion process and the thin 'tribofilm' formed during a friction mode.

Boride-based coatings for protection of cast iron against wear

“…The formed nano-size oxide layer, which may have the composition of (FeB) x O y (tribo-film), promotes ‘healing’ of the surface, as well as additional lubrication between the contacting materials, reducing the friction wear. This is very similar to the behaviour at friction wear of boronised carbon steels described earlier, 32,33 as well as of some other coatings. 46-51 Probably, elevated temperature at the testing (200°C) promotes the formation and consolidation of the oxide tribo-film.…”

“…Again, the samples prepared through the procedure 1 had smaller grooves. Assuming general similarity of the surface destruction during the friction wear under pin-on-disc conditions to the surface destruction during the friction wear under sliding rod-on-flat conditions earlier conducted with boronised steel, 32,33 it can be stated that the faster surface destruction of the boronised coatings occurs during the rather short initial time period when the surfaces asperities related to the original roughness are removed, and then the wear rate decreases. Thus, the comparative data of the surface roughness from Table 2 clearly indicate the decrease of roughness for the boronised samples after the testing, i.e.…”

“…9). As opposed to carbon steel, when COF increases fast and significantly with friction time, 32,33 the behaviour of COF with time for cast iron is steady with only slight COF rise. This probably may be explained by the presence of graphite flakes in the metal structure, which provide lubrication during the friction mode.…”

“…The top area related to FeB has hardness values greater than the bottom area related to Fe 2 B (1750-1800 and 1650-1700, respectively) that is a common point for the boronised coatings. 8,[17][18][19][20]32,33 The same level of hardness is inherent to the coatings on both cast irons A and B; there is no difference between the samples with relatively 'small' (200-250 µm) or 'large' (350-450 µm) case depth since the hardness values are defined by the formation and consolidation of the iron boride phases. At the hardness determination, the indenter was applied, of course, on the iron boride phases, not on graphite.…”

“…This failure occurs mostly through the removal of the metallic platelets formed under the plastic flow and ploughing. 10,32,[34][35][36][37] Hardness of cast iron is significantly lower than that of alumina, so wear of cast iron was rather intensive. Specific uneven 'micro-groves' of various depth on the metal surface made by the pin with hard particles can be seen under microscope (Fig.…”

Boride-based coatings for protection of cast iron against wear

“…The formed nano-size oxide layer, which may have the composition of (FeB) x O y (tribo-film), promotes ‘healing’ of the surface, as well as additional lubrication between the contacting materials, reducing the friction wear. This is very similar to the behaviour at friction wear of boronised carbon steels described earlier, 32,33 as well as of some other coatings. 46-51 Probably, elevated temperature at the testing (200°C) promotes the formation and consolidation of the oxide tribo-film.…”

“…Again, the samples prepared through the procedure 1 had smaller grooves. Assuming general similarity of the surface destruction during the friction wear under pin-on-disc conditions to the surface destruction during the friction wear under sliding rod-on-flat conditions earlier conducted with boronised steel, 32,33 it can be stated that the faster surface destruction of the boronised coatings occurs during the rather short initial time period when the surfaces asperities related to the original roughness are removed, and then the wear rate decreases. Thus, the comparative data of the surface roughness from Table 2 clearly indicate the decrease of roughness for the boronised samples after the testing, i.e.…”

“…9). As opposed to carbon steel, when COF increases fast and significantly with friction time, 32,33 the behaviour of COF with time for cast iron is steady with only slight COF rise. This probably may be explained by the presence of graphite flakes in the metal structure, which provide lubrication during the friction mode.…”

“…The top area related to FeB has hardness values greater than the bottom area related to Fe 2 B (1750-1800 and 1650-1700, respectively) that is a common point for the boronised coatings. 8,[17][18][19][20]32,33 The same level of hardness is inherent to the coatings on both cast irons A and B; there is no difference between the samples with relatively 'small' (200-250 µm) or 'large' (350-450 µm) case depth since the hardness values are defined by the formation and consolidation of the iron boride phases. At the hardness determination, the indenter was applied, of course, on the iron boride phases, not on graphite.…”

“…This failure occurs mostly through the removal of the metallic platelets formed under the plastic flow and ploughing. 10,32,[34][35][36][37] Hardness of cast iron is significantly lower than that of alumina, so wear of cast iron was rather intensive. Specific uneven 'micro-groves' of various depth on the metal surface made by the pin with hard particles can be seen under microscope (Fig.…”

Boride-based coatings for protection of cast iron against wear

“…The formed nano-size oxide layer, which may have the composition of (FeB) x O y (tribo-film), promotes ‘healing’ of the surface, as well as additional lubrication between the contacting materials, reducing the friction wear. This is very similar to the behaviour at friction wear of boronised carbon steels described earlier, 32,33 as well as of some other coatings. 46-51 Probably, elevated temperature at the testing (200°C) promotes the formation and consolidation of the oxide tribo-film.…”

“…Again, the samples prepared through the procedure 1 had smaller grooves. Assuming general similarity of the surface destruction during the friction wear under pin-on-disc conditions to the surface destruction during the friction wear under sliding rod-on-flat conditions earlier conducted with boronised steel, 32,33 it can be stated that the faster surface destruction of the boronised coatings occurs during the rather short initial time period when the surfaces asperities related to the original roughness are removed, and then the wear rate decreases. Thus, the comparative data of the surface roughness from Table 2 clearly indicate the decrease of roughness for the boronised samples after the testing, i.e.…”

“…9). As opposed to carbon steel, when COF increases fast and significantly with friction time, 32,33 the behaviour of COF with time for cast iron is steady with only slight COF rise. This probably may be explained by the presence of graphite flakes in the metal structure, which provide lubrication during the friction mode.…”

“…The top area related to FeB has hardness values greater than the bottom area related to Fe 2 B (1750-1800 and 1650-1700, respectively) that is a common point for the boronised coatings. 8,[17][18][19][20]32,33 The same level of hardness is inherent to the coatings on both cast irons A and B; there is no difference between the samples with relatively 'small' (200-250 µm) or 'large' (350-450 µm) case depth since the hardness values are defined by the formation and consolidation of the iron boride phases. At the hardness determination, the indenter was applied, of course, on the iron boride phases, not on graphite.…”

“…This failure occurs mostly through the removal of the metallic platelets formed under the plastic flow and ploughing. 10,32,[34][35][36][37] Hardness of cast iron is significantly lower than that of alumina, so wear of cast iron was rather intensive. Specific uneven 'micro-groves' of various depth on the metal surface made by the pin with hard particles can be seen under microscope (Fig.…”

Boride-based coatings for protection of cast iron against wear

“…The formed nano-size oxide layer, which may have the composition of (FeB) x O y (tribo-film), promotes ‘healing’ of the surface, as well as additional lubrication between the contacting materials, reducing the friction wear. This is very similar to the behaviour at friction wear of boronised carbon steels described earlier, 32,33 as well as of some other coatings. 46-51 Probably, elevated temperature at the testing (200°C) promotes the formation and consolidation of the oxide tribo-film.…”

“…Again, the samples prepared through the procedure 1 had smaller grooves. Assuming general similarity of the surface destruction during the friction wear under pin-on-disc conditions to the surface destruction during the friction wear under sliding rod-on-flat conditions earlier conducted with boronised steel, 32,33 it can be stated that the faster surface destruction of the boronised coatings occurs during the rather short initial time period when the surfaces asperities related to the original roughness are removed, and then the wear rate decreases. Thus, the comparative data of the surface roughness from Table 2 clearly indicate the decrease of roughness for the boronised samples after the testing, i.e.…”

“…9). As opposed to carbon steel, when COF increases fast and significantly with friction time, 32,33 the behaviour of COF with time for cast iron is steady with only slight COF rise. This probably may be explained by the presence of graphite flakes in the metal structure, which provide lubrication during the friction mode.…”

“…The top area related to FeB has hardness values greater than the bottom area related to Fe 2 B (1750-1800 and 1650-1700, respectively) that is a common point for the boronised coatings. 8,[17][18][19][20]32,33 The same level of hardness is inherent to the coatings on both cast irons A and B; there is no difference between the samples with relatively 'small' (200-250 µm) or 'large' (350-450 µm) case depth since the hardness values are defined by the formation and consolidation of the iron boride phases. At the hardness determination, the indenter was applied, of course, on the iron boride phases, not on graphite.…”

“…This failure occurs mostly through the removal of the metallic platelets formed under the plastic flow and ploughing. 10,32,[34][35][36][37] Hardness of cast iron is significantly lower than that of alumina, so wear of cast iron was rather intensive. Specific uneven 'micro-groves' of various depth on the metal surface made by the pin with hard particles can be seen under microscope (Fig.…”