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Wear behavior of self-lubricating boride layers produced on Inconel 600-alloy by laser alloying
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Wear behavior of self-lubricating boride layers produced on Inconel 600-alloy by laser alloying
T
Adam Piasecki , Mateusz Kotkowiak, Natalia Makuch, Michał Kulka ⁎
Poznan University of Technology, Institute of Materials Science and Engineering, Pl. M. Sklodowskiej-Curie 5, 60-965 Poznan, Poland
ARTICLE INFO
ABSTRACT
Keywords: Self-lubricant Calcium fluoride Nickel alloy Laser boriding Tribofilm
In this work, laser alloying has been used in order to produce the self-lubricating boride layers on Inconel 600alloy. The two-step process was used during laser alloying. First, the surface of the substrate material was coated with a paste containing amorphous boron and calcium fluoride CaF2 as a self-lubricating addition. Then, this surface was re-melted by laser beam using TRUMPF 2600 Turbo CO2 laser. The laser beam powers 1.56 kW and 1.95 kW were used for laser alloying. The re-melted zone included dendritic and globular precipitates of nickel, chromium and iron borides among the soft Ni-phase as well as CaF2 particles, locating close to the surface. The microhardness of the re-melted zone, containing solid lubricant, was lower than that of the laser-alloyed layer with only boron. Whereas the wear resistance was higher. The tribofilm was observed on the worn surfaces of laser-alloyed layers with boron and CaF2. The presence of the tribofilm reduced wear of mating parts and improved their tribological properties.
1. Introduction Wear is the main cause of around 80% exhaustion of the operational potential of machines and vehicles. In the case of mating parts, it is important to lubricate them. Conducting effective lubrication of the contact surfaces of moving parts is an effective method to counteract friction and reduce their wear. The lubricating oils, used on a large scale, pollute the environment during their production and, practically, at all the stages of their use: during transport to users, long storage, during work, as well as collection and disposal at the end of the service life. Moreover, their use is limited at elevated temperature of the mating parts. The production of self-lubricating wear-resistant surface layers containing solid lubricants can be one of the most effective and economical methods to increase the durability of machine parts and tools. Nowadays, continuous interest in the surface modification of metal alloys has been observed in order to extend the period of their application. Among the metal alloys, nickel alloys were very important. Nibased alloys were the materials which had a lot of very good properties, such as high oxidation and corrosion resistance. Because of that, these materials have been widely used in the chemical industry or in construction of jet engines and wherever a high temperature and aggressive environment existed. However, nickel alloys had some disadvantages such as very poor wear resistance which restricted limits of possible
⁎
applications. Under conditions of appreciable mechanical wear (adhesive or abrasive), Ni-based alloys had to be used with suitable wear protection. The wear resistance of materials was very often improved by increasing the hardness, because the surface layer with higher hardness was usually characterized by the better tribological properties. In recent years, there was a tendency to improve the tribological properties of Ni-based alloys by surface treatment such as: diffusion boriding [1–8] or nitriding (especially plasma nitriding) [9–11], PACVD processes [12] as well as laser boriding, i.e. laser alloying with boron [13]. Solid self-lubricants are also widely used in some application to improve the tribological properties, especially to reduce the friction coefficient and relative loss of mass. Such materials can be divided into the three groups: low-temperature solid lubricants, which can work from −200 °C (473 K) to room temperature, moderate temperature solid lubricant (from room temperature to 500 °C, i.e. 773 K) and hightemperature solid lubricants, which can work above 500 °C (773 K) [14]. Calcium fluoride CaF2 belongs to the popular and important hightemperature solid lubricants. It is characterized by density of 3.18 g/ cm3 [15,16] and melting point of 1360 °C (1633 K) [17] and crystalline structure, which provides very low shear strength and prevents adhesion [18–20]. Calcium fluoride has a very good oxidation resistance [14] and this material is characterized by high chemical and thermal stability [21–23]. It means that calcium fluoride doesn’t react with
Corresponding author. E-mail address: [email protected] (A. Piasecki).
https://doi.org/10.1016/j.wear.2018.12.026 Received 3 September 2018; Received in revised form 30 October 2018; Accepted 12 December 2018 0043-1648/ © 2018 Elsevier B.V. All rights reserved.
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Table 1 Chemical composition of Ni-based alloy [wt%]. Material
C
Si
Mn
S
Al.
Cr
Cu
Fe
Ni
Inconel 600
0.078
0.18
0.16
< 0.001
0.06
15.72
0.04
8.63
75.1
other material components [24]. Previous research concerned the alloying of 100CrMnSi6-4 bearing steel. The alloying material was composed of amorphous boron and CaF2. Such a laser treatment was successfully implemented in order to improve the tribological properties of this steel [24,25]. 2. Materials and methods Fig. 2. Scheme of two-stage process of laser alloying of Inconel®600 with boron and CaF2.
2.1. Material For investigation, Inconel®600-alloy was used. In the Table 1, the chemical composition of this alloy was shown. The dimensions of ringshaped sample were as follows: external diameter – 20 mm, internal diameter – 12 mm and height – 12 mm.
rate (vl) 2.88 m/min, laser beam diameter (d) 2 mm, and overlapping (O) 86%. They were listed in Table 2.
2.2. Laser boriding with addition of calcium fluoride CaF2
2.3. Microstructure and microhardness
The laser-boriding process was carried out as the two-stage process. At first, the external surface of a ring-shaped sample was coated with the paste. The paste was made of amorphous boron, diluted polyvinyl alcohol solution and calcium fluoride CaF2 as a self-lubricating material. In Fig. 1, the powders, used for the paste preparation, were presented. Size of boron particles which had a spheroidal form was in the range of 1–10 µm (Fig. 1a). CaF2 particles were characterized by cuboidal shape, and their size was in the range of 2–6 µm (Fig. 1b). Moreover, solid-lubricant particles formed agglomerates. The thickness of the paste (tC) was 230 µm. It was measured by the thickness gauge of coatings Positector 600 using the phenomenon of the magnetic induction and eddy currents. The second step consisted in laser treatment. The scheme of two-stage process was shown in Fig. 2. The laser beam was used for re-melting the surface with the paste. The laser treatment was carried out by the TRUMPF TLF 2600 Turbo CO2 laser of the nominal power 2.6 kW. The laser processing parameters were as follows: laser beam power (P) 1.56 kW and 1.95 kW, scanning
The optical microscope (OM) and scanning electron microscope (SEM) Tescan Vega 5135 were used for microstructure observation of polished and etched cross-section of the specimen. To prepare metallographic specimen, the samples were cut perpendicularly to the produced laser tracks. The metallographic specimens were polished with the abrasive paper of different grit size, and finally, with Al2O3. To reveal the microstructure, samples were etched with the Marble's reagent (10 g CuSO4, 50 cm3HCl, 50 cm3 H2O). Microhardness of the layers were measured by the Vickers method on the cross-section of the specimens using ZWICK 3212 B apparatus. The load was F = 0.1 kgf (approximately 0.981 N). 2.4. X-ray microanalysis and phase analysis The concentrations of basic elements were measured by PGT Avalon X-ray microanalyser with EDS using 55° take-off angle. The accelerating voltage 12 kV was applied. The contents of some elements such as
Fig. 1. Components of alloying material: amorphous boron powder (a), CaF2 powder (b).
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Table 2 Laser alloying parameters. Type of material
Composition of paste
Thickness of coating tC [µm]
Laser beam diameter d [mm]
Laser beam power P [kW]
Scanning rate vl[m/min]
Overlapping O [%]
Laser-alloyed Inconel®600 with boron and CaF2
B+CaF2+ polyvinyl alcohol, mass ratio B:CaF2=5:1
230
2
1.56
2.88
86
1.95
Fig. 4. XRD Patterns of laser-alloyed Inconel®600 alloy with B and CaF2 at laser beam power of 1.56 kW (a) and 1.95 kW (b).
Fig. 3. Scheme of the wear test.
boron, calcium, chromium, iron and nickel were analyzed. The PANalytical EMPYREAN X-ray diffractometer equipped with Cu Kα radiation was used for phase analysis of the laser-alloyed layers. 2.5. Wear tests The scheme of wear resistance test was shown in the Fig. 3. This frictional pair was made of ring-shaped specimen and sintered carbide as a counter-specimen. The chemical composition of sintered carbide S20S was shown in Table 3. The properties of this material were as follows: density 11.6 g/cm3 and hardness 1500 HV. The wear resistance test lasted 2 h with the change of a counter-specimen after 30 min. Wear tests were performed at the load of 49 N under dry friction conditions. The selection of the test parameters resulted from the previous study. The results were compared to those-obtained during the previous study of typical laser-borided layers [13]. The two methods were used for evaluation of wear resistance. First, the wear resistance was shown as a Table 3 Chemical composition of cemented carbide S20 [wt%]. Material
WC
(TiC + TaC + NbC)
Co
S20
78
14
8
Fig. 5. OM microstructure of laser-alloyed Inconel®600 alloy with boron and CaF2 at laser beam power of 1.56 kW (a) and 1.95 kW (b); 1 – re-melted zone; 2 – heat-affected zone; 3 – substrate.
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Fig. 6. SEM microstructure of the HAZ and the substrate at laser beam power of 1.56 kW (a,b) and 1.95 kW (c,d).
factor of mass wear intensity Imw. This factor was defined as a ratio of mass loss to friction surface and time and it could be calculated using the equation:
Imw =
mg m S t cm2 h
3. Result and discussion 3.1. Microstructure The phase composition of the produced laser-alloyed layers is studied. The XRD patterns are shown in Fig. 4. Based on these results, the produced layers consist of a mixture of nickel borides (Ni2B, Ni3B), chromium borides (CrB, Cr2B), iron borides Fe3B and nickel. The presence of calcium fluoride as a separate phase is also confirmed. The phase analysis is performed directly after laser alloying on the treated surface using Cu Kα radiation. The use of this radiation usually enables to analyze the depth up to 20 µm, i.e. in re-melted zone. The microstructures of laser-borided layers with the addition of calcium fluoride CaF2 are shown in Fig. 5. The obtained microstructure depends on the laser beam power used. The layers are uniform in respect of their thickness. There are observed any microcracks and pores in both layers. In the microstructure, the zones appear as follows: 1 – remelted zone (MZ), heat-affected zone (HAZ) and 3 – substrate. The identification of HAZ is difficult due to the lack of phase transformations in Inconel®600-alloy. In Fig. 6, the microstructures of HAZ for both laser-alloyed layers with boron and CaF2 are shown. The slight growth of grains is observed in the HAZ of laser-alloyed layer at the laser beam power of 1.56 kW (Fig. 6a). For the layer produced at
(1)
Where: Δm is mass loss (mg), S is friction surface (cm2), t is friction time [h]. The outer cylindrical surface of the specimen (S) was taken into consideration in order to calculate the mass wear intensity factor based on the measurements of external diameter and height of the specimen during the wear test. Secondly, the wear resistance was shown by the relative loss of mass of specimen or counter-specimen according to the equation:
mi m f m = mi mi
(2)
Where: Δm is mass loss (mg), mi is an initial mass of specimen/counterspecimen (mg), mf is a final mass of specimen/counter-specimen (mg). After the wear test, the worn surfaces of the specimen and counterspecimen were analyzed using scanning electron microscope (SEM) with EDS.
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Table 4 Average depths of the characteristic zones and dilution ratios obtained. Laser beam power P [kW]
Thickness of coating with alloying material tC [µm]
Average depth of re-melted zone dMZ [µm]
Dilution ratio DR
1.56 1.95
230
384 542
0.40 0.58
1.95 kW, smaller grains are visible in the heat-affected zone (Fig. 6c). It is caused by recrystallization of grains. The microstructures of substrate below the heat-affected zones don’t differ in both cases (Fig. 6b and 6 d). The dilution ratio (DR) depends on the thickness of the paste coating (tC) and the depth of re-melted zone (dMZ) according to the equation [26]:
DR = 1
tC dMZ
beam power, a greater dilution ratio (0.58) and a larger layer thickness of 542 µm are obtained. The detailed analysis of each laser-alloyed layer is carried out using SEM images (Figs. 7–11) and chemical compositions by EDS method (Tables 6 and 7). First, the non-etched metallographic specimens are analyzed. The observations of the produced layers in the contrast of backscattered electrons (BSE) are shown in Figs. 7a, 7c, 8a, 8c and 9. It enables to reveal the differences in microstructure despite the lack of etching. Darker phases contain more light elements. Whereas the heavier elements occur in the lighter phases. The laser-alloyed layer, produced at a higher laser beam power (1.95 kW), is characterized by fine precipitates of the dark phases (Figs. 8a and 8c), which are evenly distributed throughout the whole layer. The uneven distribution of the dark phases of various sizes is characteristic of the layer, produced at 1.56 kW (Figs. 7a, 7c). The dark phases take irregular shapes which are devoid of dendritic character. The largest clusters of the dark phase are
(3)
Where: tC is the thickness of preplaced coating (μm), and dMZ is the average depth of re-melted zone (μm). The average depth of re-melted zone dMZ and dilution ratios (DR) are shown in Table 4 for the two produced self-lubricating layers at laser beam powers: 1.56 kW and 1.95 kW. The measurements and calculations differ for both layers. For the layer, produced at higher laser
Fig. 7. SEM microstructure of laser-alloyed Inconel®600-alloy with boron and CaF2 at laser beam power of 1.56 kW based on BSE images (a, c) and SE images (b, d).
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Fig. 8. SEM microstructure of laser-alloyed Inconel®600 alloy with boron and CaF2 at laser beam power of 1.95 kW based on BSE images (a, c) and SE images (b, d).
observed along the contact of the adjacent tracks. Based on this observation and taking into account the shape of the dark phases, it can be concluded that they are firstly formed during the resolidification of the molten pool. In addition, their distribution indicates the convective motions in the molten pool during the laser alloying of Inconel®600
with boron and CaF2. The X-ray microanalysis by EDS (Table 5) proves that the high chromium content (approximately 75%) and relatively low nickel content (approximately 6%) is measured in the dark phases. In lighter areas, the nickel concentration is higher (approximately 67%). Whereas the chromium content is lower (9–13%). Moreover, the
Fig. 9. SEM microstructure in the contrast of backscattered electrons (BSE) and areas of X-ray microanalysis of laser-alloyed Inconel®600-alloy with boron and CaF2 at laser beam power of 1.56 kW (a) and 1.95 kW (b).
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Fig. 10. SEM microstructure in the contrast of secondary electrons (SE) and areas of X-ray microanalysis of laser-alloyed Inconel®600-alloy with boron and CaF2 at laser beam power of 1.56 kW.
higher calcium content is detected in the darker phases. After etching of metallographic specimens with Marble's reagent, the microstructure of layers and substrates are revealed in Figs. 7b, 7d, 8b, 8d. Images in the secondary electrons contrast (SE) are taken in the same places as the images in backscattered electrons contrast (BSE) for non-etched samples (Figs. 7 and 8) and in three representative areas throughout the whole layer (Figs. 10 and 11). The darker phases are also observed in the etched samples, but their amount is smaller. The etching of the samples and the chemical composition detection by EDS in selected areas enables to analyze their microstructure in detail. Close to the surface of the layer, produced at 1.56 kW, very fine eutectic mixture is observed. The grains of the darker phase are rich in chromium. The phases rich in nickel with dendritic structure are visible in the microstructure at larger depth of the layer. Moreover, in these areas the dark phases occur. They are surrounded by the fine lamellar eutectic mixture. The layer, produced at 1.95 kW, is characterized by another microstructure. In this case, the dendritic structure is clearly visible. Between the dendrites of the nickel-rich phases, eutectic structure surrounds the smaller grains of the dark phase rich in chromium. The fine-grained microstructure of the layer produced at a laser power of 1.95 kW is caused by a higher dilution ratio (0.56) and a higher overcooling. The rate of crystallization is a function of the nucleation rate and the rate of grain growth. A higher degree of overcooling results in a higher nucleation rate and a lower growth rate (Fig. 12). Calcium fluoride CaF2 is usually used in dry conditions. During preparing the metallographic specimen, particles of CaF2 lubricant could be rinsed out. Therefore, the amount of lubricant particles
on a metallographic specimen could be underestimated. Then the fracture of the surface layers and substrates is prepared (Figs. 13 and 14). Based on the fracture observations, CaF2 appears in the form of irregular shapes. It indicates that calcium fluoride particles could be partially re-melted. The presence of solid lubricants is also confirmed by qualitative X-ray microanalysis by EDS method (see Table 8). The Ca content in CaF2 particles is in the range of 11.04–35.61%. The small atomic mass of calcium and hence, the low accelerating voltage, influences the concentrations of other elements. This voltage affects the size of the zone from which the chemical composition is examined by the EDS method. Fig. 14 shows the SE images of calcium fluoride particles that occur directly close to the surface together with a linear profile of calcium content. The increase in Ca concentration in the light particles is clearly visible. 3.2. Microhardness The microhardness profiles of laser-borided layers with the addition of calcium fluoride CaF2, produced at various laser beam powers, are shown in Fig. 15. The results are compared to the previous study [13] regarding the laser-alloyed layers with boron only at the same parameters. The typical laser-borided layers [13] were characterized by very high hardness close to the surface, obtaining about 1700 HV0.1 and about 1500 HV0.1 at 1.56 kW and 1.95 kW, respectively. If the laser beam power is equal to 1.56 kW, the hardness decreases to 1430 HV0.1 at the end of the re-melted zone, and next, to about 260–310 HV0.1 in the heat-affected zone and in the substrate. The hardness of
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Fig. 11. SEM microstructure in the contrast of secondary electrons (SE) and areas of X-ray microanalysis of laser-alloyed Inconel®600-alloy with boron and CaF2 at laser beam power of 1.95 kW.
Table 5 EDS X-ray microanalysis of Inconel®600-alloy after laser alloying with boron and CaF2 according to spots in Fig. 9. Spot
1 2 3 4 5 6 7 8
Table 6 EDS X-ray microanalysis of Inconel®600-alloy after laser alloying with boron and CaF2 at laser beam power of 1.56 kW according to spots in Fig. 10.
Element. wt%
Spot
B
Ca
Cr
Fe
Ni
12.99 11.27 13.99 11.97 13.44 17.96 15.04 13.72
0.16 0.04 0.14 0.03 0.13 0.01 0.18 0.04
75.36 13.19 49.19 14.28 78.56 8.91 63.51 11.47
5.04 7.35 7.1 7.48 2.94 5.79 3.52 9.05
6.45 68.15 29.58 66.23 4.92 67.33 17.75 65.71
1a 1b 1c 1d 2a 2b 2c 2d 3a 3b 3c 3d
laser-alloyed layer with boron at 1.95 kW decreases to 800 HV0.1 at the end of the MZ. In the HAZ and in the substrate, the hardness doesn’t differ, obtaining approximately 220 HV0.1. The hardness of the laseralloyed layers with boron and CaF2 is significantly lower. The area, marked as 1 in Fig. 10 and located close to the surface, is characterized by the hardness of about 1250 HV0.1 after process, carried out at 1.56 kW. The hardness in area, marked as 3, ranges from 1001 to 1139 HV0.1. Then, at the end of the re-melted zone the hardness decreases to 400 HV 0.1 in the area marked as 3 in Fig. 10. In the heat-affected zone, the hardness is constant and is characteristic of the base material. The
Element. wt% B
Ca
Cr
Fe
Ni
– 20.09 13.73 3.35 12.54 11.58 13.62 12.17 12.35 10.17 11.82 –
50.38 1.15 0.1 0.31 0.1 0.03 0.09 0.08 0.19 0.07 0.01 –
31.58 10.77 74.21 14.94 13.87 13.49 67.43 19.23 74.2 12.33 11.53 16.58
4.9 6.39 4.23 7.41 6.81 6.99 4.38 6.14 3.44 9.7 6.43 8.81
13.13 61.6 7.73 73.98 66.68 67.91 14.48 62.37 9.82 67.73 70.21 74.61
similar course of the microhardness profile is observed if the laser beam power of 1.95 kW is applied. The diminished microhardness (in the range of 754–870 HV0.1) is measured in MZ in the areas marked as 1and 3 in Fig. 11. Then, the hardness decreases to about 500 HV0.1 at the end of the laser re-melted zone, i.e. in the area marked as 3 in Fig. 11. The hardness of the HAZ and the substrate is similar, obtaining about 180HV0.1. Evenly distribution of hardness indicates a regular
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the decrease in microhardness of layers as well as the reduced hardness gradient between the MZ and the substrate. It could be advantageous from the point of view of the distribution of residual stresses.
Table 7 EDS X-ray microanalysis of Inconel®600 alloy after laser alloying with boron and CaF2 at laser beam power of 1.95 kW according to spots in Fig. 11. Spot
1a 1b 1c 1d 2a 2b 2c 2d 3a 3b 3c 3d
Element. wt% B
Ca
Cr
Fe
Ni
– 13.04 14.89 14.35 7.94 9.19 12.27 11.9 10.89 11.45 11.49 –
37.25 0.12 0.1 0.1 0.09 0.01 0.16 0.01 0.04 0.03 0.04 –
50.48 13.25 13.51 11.92 15.27 12.65 67.85 9.89 13.92 8.98 10.11 15.81
2.49 8.84 5.15 4.93 9.95 9.86 4.2 5.3 8.51 4.99 5.29 8.83
9.78 64.75 66.35 68.7 66.75 68.29 15.52 72.9 66.64 74.55 73.07 75.36
3.3. Wear resistance The mass loss of the samples per a unit of friction surface vs. time of friction is presented in Figs. 16a and 16b. The measurements of mass (m) were performed with the accuracy 0.01 mg. The measuring accuracy of diameter (d) and height (h) of the samples were equal to 0.001 cm. The maximal measuring errors didn’t exceed the measuring accuracy. Therefore, taking the maximal mass loss Δm = 51.78 mg into consideration, the maximal measuring errors of Δm/S values didn’t exceed 0.012 mg cm−2. These errors were smaller than the size of test points marked in the Fig. 16 and couldn’t be shown in this figure. The evaluation by the factor of mass wear intensity Imw confirmed the advantageous influence of solid lubricant on wear behavior of laser-alloyed Inconel®600-alloy. The self-lubricating layers are characterized by significantly lower factors of mass wear intensity compared to the layers, alloyed with only boron, irrespective of the laser beam power used (1.56 and 1.95 kW). In spite of much higher hardness of the typical laser-borided layers, the use of CaF2 as an additional alloying material significantly reduced the wear of the surface layers. The smallest mass wear intensity factor (Imw=1.88 mg cm2 h−1) is calculated for the selflubricating layer produced at the laser beam power of 1.95 kW, i.e. for the layer of the lowest hardness. The self-lubricating layer, produced at 1.56 kW, is characterized by slightly higher mass wear intensity factor (Imw=1.96 mg cm−2 h−1). The factor of mass wear intensity (Imw) is defined as a ratio of mass loss to friction surface and time. It represents the wear behavior during the mid-age period where a steady rate of wear occurs. This rate of wear corresponds to Imw value which is calculated as a slope of a straight line in the coordinate system: Δm/S – friction time (t). The lower value of Imw testifies that the less intensive wear is obtained. The previous research showed that the use of load less than 49 N was not advisable because of much longer duration of the primary stage of wear (early grinding-in period, also called run-in period), where surfaces adapted to each other as well as because of the relatively low wear during the secondary stage of wear (mid-age period). The position of wear diagrams for the layer with the addition of CaF2 above the wear diagrams for typical laser-borided layer (Figs. 16a and 16b) indicates only the higher wear during grinding-in period (the first half hour). The second method of wear resistance evaluation consisted in measurements of the relative mass loss Δm/mi (Figs. 16c and 16d). The time of grinding-in was omitted in the calculations of Δm/mi values.
Fig. 12. The effect of overcooling on the cooling rate [27].
phase composition of laser-alloyed layer with boron and CaF2 at 1.95 kW. The higher hardness of each layer corresponds to the presence of chromium (CrB and Cr2B) and nickel (Ni2B and Ni3B) borides in the re-melted zone. The addition of soft calcium fluoride particles causes
Fig. 13. Fractures of laser-alloyed layers with boron and CaF2; 1.56 kW (a), 1.95 kW (b).
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Fig. 14. Linear X-ray microanalysis of laser-alloyed layers with boron and CaF2 at 1.56 kW (a) and 1.95 kW (b).
Hence, the mass loss after 0.5 h of wear test was accepted as an initial mass loss (mi). In this case, the results were comparable to the evaluation of wear resistance using the mass wear intensity factor (Imw). If the calcium fluoride is added to the alloying material, the diminished values of Δm/mi are characteristic of both the specimen (laser-alloyed Inconel®600-alloy) and counter-specimen (sintered carbide S20S). It is caused by the formation of the tribofilm on the worn surface. This tribofilm protected the friction pair against wear and reduced the loss of mass. The worn surfaces of the samples are analyzed using SEM images in the BSE contrast (Fig. 17) as well as in the SE contrast with X-ray microanalysis by EDS patterns of some elements like calcium, chromium, nickel and iron (Fig. 18). The smeared tribofilm, consisting of CaF2, as well as the particles of this lubricant are clearly visible on these surfaces. The presence of tribolfilm on the worn surfaces is clearly visible, especially, in BSE images (Fig. 17). Darker areas on the worn surfaces contained light elements as calcium and fluorine. Based on the diversified grayscale, the different thickness of the tribofilm is recognized. Darker areas corresponded to the tribofilm of larger thickness. It is confirmed by the EDS analysis (Fig. 18). The measurements at relatively low accelerating voltage (12 kV) is used in order to reduce the depth of the electron interaction. On the worn surfaces of both specimens with solid lubricant, the increased calcium concentration is detected because of the formation of tribofilm. After the wear tests, the fractures are prepared to estimate the thickness of tribofilm for both specimens. These fractures are observed using SEM (Fig. 19). The largest thickness of the tribofilm obtained approximately 3 µm, regardless of the parameters of the laser alloying. In Fig. 20, the mechanism of tribofilm formation on the surface layers is shown. The three stages of self-lubrication mechanism are distinguished: grinding-in (Figs. 20b, 20c), uncovering the lubricant particles and smearing the lubricant on the worn surface (Fig. 20d), and finally, appearance of the tribofilm of diversified thickness (Fig. 20e).
Table 8 EDS X-ray microanalysis of Inconel®600-alloy after laser alloying with boron and CaF2 according to spots in Fig. 13. Spot
1 2 3 4 5 6 7 8 9
Element. wt% B
Ca
Cr
Fe
Ni
– 14.27 17.2 – – 15.36 – 21.19 –
35.61 0.18 0.1 17.98 7.08 0.12 14.30 0.09 11.04
27.90 10.98 10.62 13.44 16.47 13.72 16.51 12.82 16.03
5.09 6.42 6.11 7.26 8.12 7.05 8.51 7.02 8.38
31.40 68.15 65.97 61.32 68.33 63.76 60.68 58.88 64.55
4. Conclusions The obtained laser-alloyed layers with solid lubricant were uniform in respect of their thickness because of the high overlapping of multiple laser tracks (86%). No cracks and gas pores were observed. In both layers, produced at various laser beam powers, the three zones were visible in the microstructure: laser-re-melted zone (MZ), heat-affected zone (HAZ) and the base material (alloyed material). The grains’ growth was observed in the HAZ of layer produced at lower laser beam
Fig. 15. Microhardness profiles of laser-alloyed layers with boron only [13] and laser-alloyed layers with boron and CaF2, produced at 1.56 kW (a) and 1.95 kW (b).
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Fig. 16. Results of wear tests of laser-alloyed layers with boron only [13] and laser-alloyed layer with boron and CaF2 at laser beam power of 1.56 kW (a, c) and 1.95 kW (b, d).
Fig. 17. The surface of specimens after wear test in the contrast of backscattered electrons; laser-alloyed Inconel®600-alloy with boron and CaF2 at laser beam power of 1.56 kW (a) and 1.95 kW (b).
power of 1.56 kW. In the HAZ of self-lubricating layer produced at 1.95 kW, the decrease in the size of grains was noticed. The recrystallization of grains in this zone during formation of subsequent laser tracks was the probable reason for such a situation. At the laser beam power of 1.56 kW, the temperature was too low for recrystallization. Of course, in the case of the layer, produced at the power of 1.96 kW, there was also grains’ growth, but the temperature increased as a consequence of the formation of subsequent laser tracks. Probably, this temperature was sufficient to cause recrystallization and enabled to obtain the smaller grains in the HAZ.
The microstructure of MZ consisted of nickel, chromium and iron borides (Ni3B, Ni2B, CrB, Cr2B, Fe3B) and Ni–Cr–Fe-matrix (re-melted substrate) as well as CaF2 particles. The microstructure of the self-lubricating layer depended on the laser beam power. The layer, produced at a higher laser beam power (1.95 kW) was characterized by smaller precipitates of the dark phase, identified as CrB and Cr2B borides, which were evenly distributed in the layer. The uneven distribution of the dark phases of various sizes was observed for the layer produced at 1.56 kW. In the re-melted zones, apart from irregular particles of chromium borides, the dendritic and globular precipitates of nickel,
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Fig. 18. Worn surfaces of laser-alloyed Inconel®600 with boron and CaF2. EDS patterns of calcium, chromium, nickel and iron.
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Fig. 19. The thickness of tribofilm; laser-alloyed Inconel 600-alloy with boron and CaF2 at laser beam power of 1.56 kW (a) and 1.95 kW (b).
Fig. 20. Scheme of tribofilm formation: scheme of the wear test (a), initial stage consisting in grinding-in (b,c), uncovering the lubricant particles and smearing the lubricant on the surface of specimen (d), formation of tribolfilm of diversified thickness (e).
wear intensity factors: 1.96 and 1.88 mg cm−2 h−1, for the layer produced at 1.56 kW and 1.95 kW, respectively. The advantageous influence of solid lubricants on the wear resistance was also confirmed by the measurements of relative mass loss (Δm/mi). The solid lubricant particles caused the formation of the tribofilm on the worn surfaces of laser-alloyed layers with boron and CaF2. The tribofilm was characterized by diversified thickness, obtaining up to approximately 3 µm. The presence of the tribofilm reduced wear of mating parts and improved their tribological properties. The mechanism of laser alloying with solid lubricants was developed (Fig. 21), showing the formation of re-melted and heat-affected zones, convection and vortex motions in molten pool and temperature distribution during laser treatment.
chromium and iron borides were visible among the soft Ni-phase as well as CaF2 particles, locating close to the surface. During laser alloying, these particles were partially melted and vaporized, partially melted and re-solidified. The self-lubricating layer, produced at a higher laser beam power, was characterized by a higher dilution ratio (0.56) and layer thickness (542 µm) compared to the layer produced at 1.56 kW (DR=0.4, tMZ=384 µm). The significantly lower hardness was characteristic of the self-lubricating layers in comparison with the laser-alloyed layers with boron only. In spite of that, the significant increase in wear resistance of laseralloyed layers with boron and CaF2 was observed. The self-lubricating layers were more resistant to wear, obtaining the lower values of mass
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Fig. 21. Scheme of the two-stage process of laser alloying of Inconel®600 with boron and CaF2.
Acknowledgements
[10] Y. Sun, Kinetics of layer growth during plasma nitriding of nickel based alloy Inconel 600, J. Alloy. Compd. 351 (2003) 241–247. [11] M. Kost, J. Rudnicki, T. Wierzchoń, Glow discharge treatment of Inconel 625 for glass fibreizing spinner discs, Proc. Insti. Mech. Eng. Part L: J. Mater.: Des. Appl. 222 (4) (2008) 241–243. [12] R. Sitek, K. Sikorski, J. Sobczak, T. Wierzchoń, Structure and properties of the multilayers produced on Inconel 600 by the PACVD method with the participation of trimethylaluminum vapours, Mater. Sci. 26 (3) (2008) 767–777. [13] M. Kulka, P. Dziarski, N. Makuch, A. Piasecki, A. Miklaszewski, Microstructure and properties of laser-borided Inconel 600-alloy, Appl. Surf. Sci. 284 (2013) 757–771. [14] J.-F. Yang, Y. Jiang, J. Hardell, B. Prakash, Q.-F. Fang, Influence of service temperature on tribological characteristics of self-lubricant coatings: a review, Front. Mater. Sci. 7 (1) (2013) 28–39. [15] S. Peilong, Y. Xuefeng, W. Shouren, Y. Liying, Tribological properties of self-lubricating laminated ceramic materials, J. Wuhan. Univ. Technol.-Mater. Sci. Ed. (2014) 906–911. [16] T. Kobayashi, T. Maruyama, T. Yasuda, Sliding properties of composite sprayed coating between bronze powder and solid lubricant, Mater. Trans. 44 (5) (2003) 1024–1028. [17] Z.-F. Xiang, X.-B. Liu, J. Ren, J. Luo, S.-H. Shi, Y. Chen, G.-L. Shi, S.-H. Wu, Investigation of laser cladding high temperature anti-wear composite coatings on Ti6Al4V alloy with the addition of self-lubricant CaF2, Appl. Surf. Sci. 313 (2014) 243–250. [18] H. Yan, P. Zhang, Z. Yu, Q. Lu, S. Yang, C. Li, Microstructure and tribological properties of laser-clad Ni–Cr/TiB2 composite coatings on copper with the addition of CaF2, Surf. Coat. Technol. 206 (2012) 4046–4053. [19] W.-G. Liu, X.-B. Liu, Z.-G. Zhang, J. Guo, Development and characterization of composite Ni–Cr–C–CaF2 laser cladding on TiAl matrc alloy, J. Alloy. Compd. 470 (2009) 25–28. [20] H.M. Wang, Y.L. Yu, S.Q. Li, Microstructure and tribological properties of laser clad CaF2/Al2O3 self-lubrication wear-resistant ceramic matrix composite coatings, Scr. Mater. 47 (2002) 57–61. [21] X. Zhang, L. Zhang, Z. Huang, Characterization of Ni-based alloy submicron WS2/ CaF2 composite coatings deposited by high velocity oxy-fuel (HVOF) spray process, Adv. Mater. Res. 881–883 (2014) 1407–1411. [22] J. Luo, X.-B. Liu, Z.-F. Xiang, S.-H. Shi, Y. Chen, G.-L. Shi, S.-H. Wu, Y.-N. Wu,
This work has been financially supported by Ministry of Science and Higher Education in Poland as a part of the 02/24/DSPB project. The authors wish to thank Ph.D. D.Sci. P. Twardowski and Ph.D. D.Sci. D. Przestacki from Division of Machining for their help and cooperation during the laser treatment and M.Sc.Eng. J. Jakubowski from Division of Metal Science and Surface Engineering for his help and cooperation during the realization of this work. References [1] I. Ozbek, H. Akbulut, S. Zeytin, C. Bindal, A. Hikmet Ucisik, The characterization of borided 99.5% purity nickel, Surf. Coat. Technol. 126 (2000) 166–170. [2] D. Mu, B. Shen, C. Yang, X. Zhao, Microstructure analysis of boronized pure nickel using boronizing powders with SiC as diluent, Vacuum 83 (2009) 1481–1484. [3] W. Muhammad, K. Hussain, A. Tauqir, A. Ul Haq, A.Q. Khan, Evaluation of halideactivated pack boriding of INCONEL 722, Metall. Mater. Trans. A 30A (1999) 670–675. [4] K.G. Anthymidis, P. Zinoviadis, D. Roussos, D.N. Tsipas, Boriding of nickel in a fluidized bed reactor, Mater. Res Bull. 37 (2002) 515–522. [5] N. Ueda, T. Mizukoshi, K. Demizu, T. Sone, A. Ikenaga, M. Kawamoto, Boriding of nickel by the powder-pack method, Surf. Coat. Technol. 126 (2000) 25–30. [6] R.S. Petrova, N. Suwattananont, V. Samardzic, The effect of boronizing on metallic alloys for automotive applications, J. Mater. Eng. Perform. 17 (3) (2008) 340–345. [7] D.C. Lou, J.K. Solberg, O.M. Akselsen, N. Dahl, Microstructure and property investigation of paste boronized pure nickel and Nimonic 90 superalloy, Mater. Chem. Phys. 115 (2009) 239–244. [8] O. Torun, I. Çelikyürek, Boriding of diffusion bonded joints of pure nickel to commercially pure titanium, Mater. Des. 30 (2009) 1830–1834. [9] P.K. Aw, A.W. Batchelor, N.L. Loh, Structure and tribological properties of plasma nitrided surface films on Inconel 718, Surf. Coat. Technol. 89 (1997) 70–76.
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(2016) 173–191. [25] A. Piasecki, M. Kotkowiak, M. Kulka, Self-lubricating surface layers produced using laser alloying of bearing steel, Wear 376–377 (2017) 993–1008. [26] C.T. Kwok, F.T. Cheng, H.C. Man, Laser-fabricated Fe–Ni–Co–Cr–B austenitic alloy on steels. Part I. Microstruct cavitation, Eros Behav. Surf. Coat. Technol. 145 (2001) 194–205. [27] M. Blicharski, Wstep do inzynierii materialowej/Introduction to materials engineering, in: Polish, WNT, 2012.
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Wear behavior of self-lubricating boride layers produced on Inconel 600-alloy by laser alloying
T
Adam Piasecki , Mateusz Kotkowiak, Natalia Makuch, Michał Kulka ⁎
Poznan University of Technology, Institute of Materials Science and Engineering, Pl. M. Sklodowskiej-Curie 5, 60-965 Poznan, Poland
ARTICLE INFO
ABSTRACT
Keywords: Self-lubricant Calcium fluoride Nickel alloy Laser boriding Tribofilm
In this work, laser alloying has been used in order to produce the self-lubricating boride layers on Inconel 600alloy. The two-step process was used during laser alloying. First, the surface of the substrate material was coated with a paste containing amorphous boron and calcium fluoride CaF2 as a self-lubricating addition. Then, this surface was re-melted by laser beam using TRUMPF 2600 Turbo CO2 laser. The laser beam powers 1.56 kW and 1.95 kW were used for laser alloying. The re-melted zone included dendritic and globular precipitates of nickel, chromium and iron borides among the soft Ni-phase as well as CaF2 particles, locating close to the surface. The microhardness of the re-melted zone, containing solid lubricant, was lower than that of the laser-alloyed layer with only boron. Whereas the wear resistance was higher. The tribofilm was observed on the worn surfaces of laser-alloyed layers with boron and CaF2. The presence of the tribofilm reduced wear of mating parts and improved their tribological properties.
1. Introduction Wear is the main cause of around 80% exhaustion of the operational potential of machines and vehicles. In the case of mating parts, it is important to lubricate them. Conducting effective lubrication of the contact surfaces of moving parts is an effective method to counteract friction and reduce their wear. The lubricating oils, used on a large scale, pollute the environment during their production and, practically, at all the stages of their use: during transport to users, long storage, during work, as well as collection and disposal at the end of the service life. Moreover, their use is limited at elevated temperature of the mating parts. The production of self-lubricating wear-resistant surface layers containing solid lubricants can be one of the most effective and economical methods to increase the durability of machine parts and tools. Nowadays, continuous interest in the surface modification of metal alloys has been observed in order to extend the period of their application. Among the metal alloys, nickel alloys were very important. Nibased alloys were the materials which had a lot of very good properties, such as high oxidation and corrosion resistance. Because of that, these materials have been widely used in the chemical industry or in construction of jet engines and wherever a high temperature and aggressive environment existed. However, nickel alloys had some disadvantages such as very poor wear resistance which restricted limits of possible
⁎
applications. Under conditions of appreciable mechanical wear (adhesive or abrasive), Ni-based alloys had to be used with suitable wear protection. The wear resistance of materials was very often improved by increasing the hardness, because the surface layer with higher hardness was usually characterized by the better tribological properties. In recent years, there was a tendency to improve the tribological properties of Ni-based alloys by surface treatment such as: diffusion boriding [1–8] or nitriding (especially plasma nitriding) [9–11], PACVD processes [12] as well as laser boriding, i.e. laser alloying with boron [13]. Solid self-lubricants are also widely used in some application to improve the tribological properties, especially to reduce the friction coefficient and relative loss of mass. Such materials can be divided into the three groups: low-temperature solid lubricants, which can work from −200 °C (473 K) to room temperature, moderate temperature solid lubricant (from room temperature to 500 °C, i.e. 773 K) and hightemperature solid lubricants, which can work above 500 °C (773 K) [14]. Calcium fluoride CaF2 belongs to the popular and important hightemperature solid lubricants. It is characterized by density of 3.18 g/ cm3 [15,16] and melting point of 1360 °C (1633 K) [17] and crystalline structure, which provides very low shear strength and prevents adhesion [18–20]. Calcium fluoride has a very good oxidation resistance [14] and this material is characterized by high chemical and thermal stability [21–23]. It means that calcium fluoride doesn’t react with
Corresponding author. E-mail address: [email protected] (A. Piasecki).
https://doi.org/10.1016/j.wear.2018.12.026 Received 3 September 2018; Received in revised form 30 October 2018; Accepted 12 December 2018 0043-1648/ © 2018 Elsevier B.V. All rights reserved.
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Table 1 Chemical composition of Ni-based alloy [wt%]. Material
C
Si
Mn
S
Al.
Cr
Cu
Fe
Ni
Inconel 600
0.078
0.18
0.16
< 0.001
0.06
15.72
0.04
8.63
75.1
other material components [24]. Previous research concerned the alloying of 100CrMnSi6-4 bearing steel. The alloying material was composed of amorphous boron and CaF2. Such a laser treatment was successfully implemented in order to improve the tribological properties of this steel [24,25]. 2. Materials and methods Fig. 2. Scheme of two-stage process of laser alloying of Inconel®600 with boron and CaF2.
2.1. Material For investigation, Inconel®600-alloy was used. In the Table 1, the chemical composition of this alloy was shown. The dimensions of ringshaped sample were as follows: external diameter – 20 mm, internal diameter – 12 mm and height – 12 mm.
rate (vl) 2.88 m/min, laser beam diameter (d) 2 mm, and overlapping (O) 86%. They were listed in Table 2.
2.2. Laser boriding with addition of calcium fluoride CaF2
2.3. Microstructure and microhardness
The laser-boriding process was carried out as the two-stage process. At first, the external surface of a ring-shaped sample was coated with the paste. The paste was made of amorphous boron, diluted polyvinyl alcohol solution and calcium fluoride CaF2 as a self-lubricating material. In Fig. 1, the powders, used for the paste preparation, were presented. Size of boron particles which had a spheroidal form was in the range of 1–10 µm (Fig. 1a). CaF2 particles were characterized by cuboidal shape, and their size was in the range of 2–6 µm (Fig. 1b). Moreover, solid-lubricant particles formed agglomerates. The thickness of the paste (tC) was 230 µm. It was measured by the thickness gauge of coatings Positector 600 using the phenomenon of the magnetic induction and eddy currents. The second step consisted in laser treatment. The scheme of two-stage process was shown in Fig. 2. The laser beam was used for re-melting the surface with the paste. The laser treatment was carried out by the TRUMPF TLF 2600 Turbo CO2 laser of the nominal power 2.6 kW. The laser processing parameters were as follows: laser beam power (P) 1.56 kW and 1.95 kW, scanning
The optical microscope (OM) and scanning electron microscope (SEM) Tescan Vega 5135 were used for microstructure observation of polished and etched cross-section of the specimen. To prepare metallographic specimen, the samples were cut perpendicularly to the produced laser tracks. The metallographic specimens were polished with the abrasive paper of different grit size, and finally, with Al2O3. To reveal the microstructure, samples were etched with the Marble's reagent (10 g CuSO4, 50 cm3HCl, 50 cm3 H2O). Microhardness of the layers were measured by the Vickers method on the cross-section of the specimens using ZWICK 3212 B apparatus. The load was F = 0.1 kgf (approximately 0.981 N). 2.4. X-ray microanalysis and phase analysis The concentrations of basic elements were measured by PGT Avalon X-ray microanalyser with EDS using 55° take-off angle. The accelerating voltage 12 kV was applied. The contents of some elements such as
Fig. 1. Components of alloying material: amorphous boron powder (a), CaF2 powder (b).
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Table 2 Laser alloying parameters. Type of material
Composition of paste
Thickness of coating tC [µm]
Laser beam diameter d [mm]
Laser beam power P [kW]
Scanning rate vl[m/min]
Overlapping O [%]
Laser-alloyed Inconel®600 with boron and CaF2
B+CaF2+ polyvinyl alcohol, mass ratio B:CaF2=5:1
230
2
1.56
2.88
86
1.95
Fig. 4. XRD Patterns of laser-alloyed Inconel®600 alloy with B and CaF2 at laser beam power of 1.56 kW (a) and 1.95 kW (b).
Fig. 3. Scheme of the wear test.
boron, calcium, chromium, iron and nickel were analyzed. The PANalytical EMPYREAN X-ray diffractometer equipped with Cu Kα radiation was used for phase analysis of the laser-alloyed layers. 2.5. Wear tests The scheme of wear resistance test was shown in the Fig. 3. This frictional pair was made of ring-shaped specimen and sintered carbide as a counter-specimen. The chemical composition of sintered carbide S20S was shown in Table 3. The properties of this material were as follows: density 11.6 g/cm3 and hardness 1500 HV. The wear resistance test lasted 2 h with the change of a counter-specimen after 30 min. Wear tests were performed at the load of 49 N under dry friction conditions. The selection of the test parameters resulted from the previous study. The results were compared to those-obtained during the previous study of typical laser-borided layers [13]. The two methods were used for evaluation of wear resistance. First, the wear resistance was shown as a Table 3 Chemical composition of cemented carbide S20 [wt%]. Material
WC
(TiC + TaC + NbC)
Co
S20
78
14
8
Fig. 5. OM microstructure of laser-alloyed Inconel®600 alloy with boron and CaF2 at laser beam power of 1.56 kW (a) and 1.95 kW (b); 1 – re-melted zone; 2 – heat-affected zone; 3 – substrate.
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Fig. 6. SEM microstructure of the HAZ and the substrate at laser beam power of 1.56 kW (a,b) and 1.95 kW (c,d).
factor of mass wear intensity Imw. This factor was defined as a ratio of mass loss to friction surface and time and it could be calculated using the equation:
Imw =
mg m S t cm2 h
3. Result and discussion 3.1. Microstructure The phase composition of the produced laser-alloyed layers is studied. The XRD patterns are shown in Fig. 4. Based on these results, the produced layers consist of a mixture of nickel borides (Ni2B, Ni3B), chromium borides (CrB, Cr2B), iron borides Fe3B and nickel. The presence of calcium fluoride as a separate phase is also confirmed. The phase analysis is performed directly after laser alloying on the treated surface using Cu Kα radiation. The use of this radiation usually enables to analyze the depth up to 20 µm, i.e. in re-melted zone. The microstructures of laser-borided layers with the addition of calcium fluoride CaF2 are shown in Fig. 5. The obtained microstructure depends on the laser beam power used. The layers are uniform in respect of their thickness. There are observed any microcracks and pores in both layers. In the microstructure, the zones appear as follows: 1 – remelted zone (MZ), heat-affected zone (HAZ) and 3 – substrate. The identification of HAZ is difficult due to the lack of phase transformations in Inconel®600-alloy. In Fig. 6, the microstructures of HAZ for both laser-alloyed layers with boron and CaF2 are shown. The slight growth of grains is observed in the HAZ of laser-alloyed layer at the laser beam power of 1.56 kW (Fig. 6a). For the layer produced at
(1)
Where: Δm is mass loss (mg), S is friction surface (cm2), t is friction time [h]. The outer cylindrical surface of the specimen (S) was taken into consideration in order to calculate the mass wear intensity factor based on the measurements of external diameter and height of the specimen during the wear test. Secondly, the wear resistance was shown by the relative loss of mass of specimen or counter-specimen according to the equation:
mi m f m = mi mi
(2)
Where: Δm is mass loss (mg), mi is an initial mass of specimen/counterspecimen (mg), mf is a final mass of specimen/counter-specimen (mg). After the wear test, the worn surfaces of the specimen and counterspecimen were analyzed using scanning electron microscope (SEM) with EDS.
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Table 4 Average depths of the characteristic zones and dilution ratios obtained. Laser beam power P [kW]
Thickness of coating with alloying material tC [µm]
Average depth of re-melted zone dMZ [µm]
Dilution ratio DR
1.56 1.95
230
384 542
0.40 0.58
1.95 kW, smaller grains are visible in the heat-affected zone (Fig. 6c). It is caused by recrystallization of grains. The microstructures of substrate below the heat-affected zones don’t differ in both cases (Fig. 6b and 6 d). The dilution ratio (DR) depends on the thickness of the paste coating (tC) and the depth of re-melted zone (dMZ) according to the equation [26]:
DR = 1
tC dMZ
beam power, a greater dilution ratio (0.58) and a larger layer thickness of 542 µm are obtained. The detailed analysis of each laser-alloyed layer is carried out using SEM images (Figs. 7–11) and chemical compositions by EDS method (Tables 6 and 7). First, the non-etched metallographic specimens are analyzed. The observations of the produced layers in the contrast of backscattered electrons (BSE) are shown in Figs. 7a, 7c, 8a, 8c and 9. It enables to reveal the differences in microstructure despite the lack of etching. Darker phases contain more light elements. Whereas the heavier elements occur in the lighter phases. The laser-alloyed layer, produced at a higher laser beam power (1.95 kW), is characterized by fine precipitates of the dark phases (Figs. 8a and 8c), which are evenly distributed throughout the whole layer. The uneven distribution of the dark phases of various sizes is characteristic of the layer, produced at 1.56 kW (Figs. 7a, 7c). The dark phases take irregular shapes which are devoid of dendritic character. The largest clusters of the dark phase are
(3)
Where: tC is the thickness of preplaced coating (μm), and dMZ is the average depth of re-melted zone (μm). The average depth of re-melted zone dMZ and dilution ratios (DR) are shown in Table 4 for the two produced self-lubricating layers at laser beam powers: 1.56 kW and 1.95 kW. The measurements and calculations differ for both layers. For the layer, produced at higher laser
Fig. 7. SEM microstructure of laser-alloyed Inconel®600-alloy with boron and CaF2 at laser beam power of 1.56 kW based on BSE images (a, c) and SE images (b, d).
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Fig. 8. SEM microstructure of laser-alloyed Inconel®600 alloy with boron and CaF2 at laser beam power of 1.95 kW based on BSE images (a, c) and SE images (b, d).
observed along the contact of the adjacent tracks. Based on this observation and taking into account the shape of the dark phases, it can be concluded that they are firstly formed during the resolidification of the molten pool. In addition, their distribution indicates the convective motions in the molten pool during the laser alloying of Inconel®600
with boron and CaF2. The X-ray microanalysis by EDS (Table 5) proves that the high chromium content (approximately 75%) and relatively low nickel content (approximately 6%) is measured in the dark phases. In lighter areas, the nickel concentration is higher (approximately 67%). Whereas the chromium content is lower (9–13%). Moreover, the
Fig. 9. SEM microstructure in the contrast of backscattered electrons (BSE) and areas of X-ray microanalysis of laser-alloyed Inconel®600-alloy with boron and CaF2 at laser beam power of 1.56 kW (a) and 1.95 kW (b).
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Fig. 10. SEM microstructure in the contrast of secondary electrons (SE) and areas of X-ray microanalysis of laser-alloyed Inconel®600-alloy with boron and CaF2 at laser beam power of 1.56 kW.
higher calcium content is detected in the darker phases. After etching of metallographic specimens with Marble's reagent, the microstructure of layers and substrates are revealed in Figs. 7b, 7d, 8b, 8d. Images in the secondary electrons contrast (SE) are taken in the same places as the images in backscattered electrons contrast (BSE) for non-etched samples (Figs. 7 and 8) and in three representative areas throughout the whole layer (Figs. 10 and 11). The darker phases are also observed in the etched samples, but their amount is smaller. The etching of the samples and the chemical composition detection by EDS in selected areas enables to analyze their microstructure in detail. Close to the surface of the layer, produced at 1.56 kW, very fine eutectic mixture is observed. The grains of the darker phase are rich in chromium. The phases rich in nickel with dendritic structure are visible in the microstructure at larger depth of the layer. Moreover, in these areas the dark phases occur. They are surrounded by the fine lamellar eutectic mixture. The layer, produced at 1.95 kW, is characterized by another microstructure. In this case, the dendritic structure is clearly visible. Between the dendrites of the nickel-rich phases, eutectic structure surrounds the smaller grains of the dark phase rich in chromium. The fine-grained microstructure of the layer produced at a laser power of 1.95 kW is caused by a higher dilution ratio (0.56) and a higher overcooling. The rate of crystallization is a function of the nucleation rate and the rate of grain growth. A higher degree of overcooling results in a higher nucleation rate and a lower growth rate (Fig. 12). Calcium fluoride CaF2 is usually used in dry conditions. During preparing the metallographic specimen, particles of CaF2 lubricant could be rinsed out. Therefore, the amount of lubricant particles
on a metallographic specimen could be underestimated. Then the fracture of the surface layers and substrates is prepared (Figs. 13 and 14). Based on the fracture observations, CaF2 appears in the form of irregular shapes. It indicates that calcium fluoride particles could be partially re-melted. The presence of solid lubricants is also confirmed by qualitative X-ray microanalysis by EDS method (see Table 8). The Ca content in CaF2 particles is in the range of 11.04–35.61%. The small atomic mass of calcium and hence, the low accelerating voltage, influences the concentrations of other elements. This voltage affects the size of the zone from which the chemical composition is examined by the EDS method. Fig. 14 shows the SE images of calcium fluoride particles that occur directly close to the surface together with a linear profile of calcium content. The increase in Ca concentration in the light particles is clearly visible. 3.2. Microhardness The microhardness profiles of laser-borided layers with the addition of calcium fluoride CaF2, produced at various laser beam powers, are shown in Fig. 15. The results are compared to the previous study [13] regarding the laser-alloyed layers with boron only at the same parameters. The typical laser-borided layers [13] were characterized by very high hardness close to the surface, obtaining about 1700 HV0.1 and about 1500 HV0.1 at 1.56 kW and 1.95 kW, respectively. If the laser beam power is equal to 1.56 kW, the hardness decreases to 1430 HV0.1 at the end of the re-melted zone, and next, to about 260–310 HV0.1 in the heat-affected zone and in the substrate. The hardness of
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Fig. 11. SEM microstructure in the contrast of secondary electrons (SE) and areas of X-ray microanalysis of laser-alloyed Inconel®600-alloy with boron and CaF2 at laser beam power of 1.95 kW.
Table 5 EDS X-ray microanalysis of Inconel®600-alloy after laser alloying with boron and CaF2 according to spots in Fig. 9. Spot
1 2 3 4 5 6 7 8
Table 6 EDS X-ray microanalysis of Inconel®600-alloy after laser alloying with boron and CaF2 at laser beam power of 1.56 kW according to spots in Fig. 10.
Element. wt%
Spot
B
Ca
Cr
Fe
Ni
12.99 11.27 13.99 11.97 13.44 17.96 15.04 13.72
0.16 0.04 0.14 0.03 0.13 0.01 0.18 0.04
75.36 13.19 49.19 14.28 78.56 8.91 63.51 11.47
5.04 7.35 7.1 7.48 2.94 5.79 3.52 9.05
6.45 68.15 29.58 66.23 4.92 67.33 17.75 65.71
1a 1b 1c 1d 2a 2b 2c 2d 3a 3b 3c 3d
laser-alloyed layer with boron at 1.95 kW decreases to 800 HV0.1 at the end of the MZ. In the HAZ and in the substrate, the hardness doesn’t differ, obtaining approximately 220 HV0.1. The hardness of the laseralloyed layers with boron and CaF2 is significantly lower. The area, marked as 1 in Fig. 10 and located close to the surface, is characterized by the hardness of about 1250 HV0.1 after process, carried out at 1.56 kW. The hardness in area, marked as 3, ranges from 1001 to 1139 HV0.1. Then, at the end of the re-melted zone the hardness decreases to 400 HV 0.1 in the area marked as 3 in Fig. 10. In the heat-affected zone, the hardness is constant and is characteristic of the base material. The
Element. wt% B
Ca
Cr
Fe
Ni
– 20.09 13.73 3.35 12.54 11.58 13.62 12.17 12.35 10.17 11.82 –
50.38 1.15 0.1 0.31 0.1 0.03 0.09 0.08 0.19 0.07 0.01 –
31.58 10.77 74.21 14.94 13.87 13.49 67.43 19.23 74.2 12.33 11.53 16.58
4.9 6.39 4.23 7.41 6.81 6.99 4.38 6.14 3.44 9.7 6.43 8.81
13.13 61.6 7.73 73.98 66.68 67.91 14.48 62.37 9.82 67.73 70.21 74.61
similar course of the microhardness profile is observed if the laser beam power of 1.95 kW is applied. The diminished microhardness (in the range of 754–870 HV0.1) is measured in MZ in the areas marked as 1and 3 in Fig. 11. Then, the hardness decreases to about 500 HV0.1 at the end of the laser re-melted zone, i.e. in the area marked as 3 in Fig. 11. The hardness of the HAZ and the substrate is similar, obtaining about 180HV0.1. Evenly distribution of hardness indicates a regular
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the decrease in microhardness of layers as well as the reduced hardness gradient between the MZ and the substrate. It could be advantageous from the point of view of the distribution of residual stresses.
Table 7 EDS X-ray microanalysis of Inconel®600 alloy after laser alloying with boron and CaF2 at laser beam power of 1.95 kW according to spots in Fig. 11. Spot
1a 1b 1c 1d 2a 2b 2c 2d 3a 3b 3c 3d
Element. wt% B
Ca
Cr
Fe
Ni
– 13.04 14.89 14.35 7.94 9.19 12.27 11.9 10.89 11.45 11.49 –
37.25 0.12 0.1 0.1 0.09 0.01 0.16 0.01 0.04 0.03 0.04 –
50.48 13.25 13.51 11.92 15.27 12.65 67.85 9.89 13.92 8.98 10.11 15.81
2.49 8.84 5.15 4.93 9.95 9.86 4.2 5.3 8.51 4.99 5.29 8.83
9.78 64.75 66.35 68.7 66.75 68.29 15.52 72.9 66.64 74.55 73.07 75.36
3.3. Wear resistance The mass loss of the samples per a unit of friction surface vs. time of friction is presented in Figs. 16a and 16b. The measurements of mass (m) were performed with the accuracy 0.01 mg. The measuring accuracy of diameter (d) and height (h) of the samples were equal to 0.001 cm. The maximal measuring errors didn’t exceed the measuring accuracy. Therefore, taking the maximal mass loss Δm = 51.78 mg into consideration, the maximal measuring errors of Δm/S values didn’t exceed 0.012 mg cm−2. These errors were smaller than the size of test points marked in the Fig. 16 and couldn’t be shown in this figure. The evaluation by the factor of mass wear intensity Imw confirmed the advantageous influence of solid lubricant on wear behavior of laser-alloyed Inconel®600-alloy. The self-lubricating layers are characterized by significantly lower factors of mass wear intensity compared to the layers, alloyed with only boron, irrespective of the laser beam power used (1.56 and 1.95 kW). In spite of much higher hardness of the typical laser-borided layers, the use of CaF2 as an additional alloying material significantly reduced the wear of the surface layers. The smallest mass wear intensity factor (Imw=1.88 mg cm2 h−1) is calculated for the selflubricating layer produced at the laser beam power of 1.95 kW, i.e. for the layer of the lowest hardness. The self-lubricating layer, produced at 1.56 kW, is characterized by slightly higher mass wear intensity factor (Imw=1.96 mg cm−2 h−1). The factor of mass wear intensity (Imw) is defined as a ratio of mass loss to friction surface and time. It represents the wear behavior during the mid-age period where a steady rate of wear occurs. This rate of wear corresponds to Imw value which is calculated as a slope of a straight line in the coordinate system: Δm/S – friction time (t). The lower value of Imw testifies that the less intensive wear is obtained. The previous research showed that the use of load less than 49 N was not advisable because of much longer duration of the primary stage of wear (early grinding-in period, also called run-in period), where surfaces adapted to each other as well as because of the relatively low wear during the secondary stage of wear (mid-age period). The position of wear diagrams for the layer with the addition of CaF2 above the wear diagrams for typical laser-borided layer (Figs. 16a and 16b) indicates only the higher wear during grinding-in period (the first half hour). The second method of wear resistance evaluation consisted in measurements of the relative mass loss Δm/mi (Figs. 16c and 16d). The time of grinding-in was omitted in the calculations of Δm/mi values.
Fig. 12. The effect of overcooling on the cooling rate [27].
phase composition of laser-alloyed layer with boron and CaF2 at 1.95 kW. The higher hardness of each layer corresponds to the presence of chromium (CrB and Cr2B) and nickel (Ni2B and Ni3B) borides in the re-melted zone. The addition of soft calcium fluoride particles causes
Fig. 13. Fractures of laser-alloyed layers with boron and CaF2; 1.56 kW (a), 1.95 kW (b).
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Fig. 14. Linear X-ray microanalysis of laser-alloyed layers with boron and CaF2 at 1.56 kW (a) and 1.95 kW (b).
Hence, the mass loss after 0.5 h of wear test was accepted as an initial mass loss (mi). In this case, the results were comparable to the evaluation of wear resistance using the mass wear intensity factor (Imw). If the calcium fluoride is added to the alloying material, the diminished values of Δm/mi are characteristic of both the specimen (laser-alloyed Inconel®600-alloy) and counter-specimen (sintered carbide S20S). It is caused by the formation of the tribofilm on the worn surface. This tribofilm protected the friction pair against wear and reduced the loss of mass. The worn surfaces of the samples are analyzed using SEM images in the BSE contrast (Fig. 17) as well as in the SE contrast with X-ray microanalysis by EDS patterns of some elements like calcium, chromium, nickel and iron (Fig. 18). The smeared tribofilm, consisting of CaF2, as well as the particles of this lubricant are clearly visible on these surfaces. The presence of tribolfilm on the worn surfaces is clearly visible, especially, in BSE images (Fig. 17). Darker areas on the worn surfaces contained light elements as calcium and fluorine. Based on the diversified grayscale, the different thickness of the tribofilm is recognized. Darker areas corresponded to the tribofilm of larger thickness. It is confirmed by the EDS analysis (Fig. 18). The measurements at relatively low accelerating voltage (12 kV) is used in order to reduce the depth of the electron interaction. On the worn surfaces of both specimens with solid lubricant, the increased calcium concentration is detected because of the formation of tribofilm. After the wear tests, the fractures are prepared to estimate the thickness of tribofilm for both specimens. These fractures are observed using SEM (Fig. 19). The largest thickness of the tribofilm obtained approximately 3 µm, regardless of the parameters of the laser alloying. In Fig. 20, the mechanism of tribofilm formation on the surface layers is shown. The three stages of self-lubrication mechanism are distinguished: grinding-in (Figs. 20b, 20c), uncovering the lubricant particles and smearing the lubricant on the worn surface (Fig. 20d), and finally, appearance of the tribofilm of diversified thickness (Fig. 20e).
Table 8 EDS X-ray microanalysis of Inconel®600-alloy after laser alloying with boron and CaF2 according to spots in Fig. 13. Spot
1 2 3 4 5 6 7 8 9
Element. wt% B
Ca
Cr
Fe
Ni
– 14.27 17.2 – – 15.36 – 21.19 –
35.61 0.18 0.1 17.98 7.08 0.12 14.30 0.09 11.04
27.90 10.98 10.62 13.44 16.47 13.72 16.51 12.82 16.03
5.09 6.42 6.11 7.26 8.12 7.05 8.51 7.02 8.38
31.40 68.15 65.97 61.32 68.33 63.76 60.68 58.88 64.55
4. Conclusions The obtained laser-alloyed layers with solid lubricant were uniform in respect of their thickness because of the high overlapping of multiple laser tracks (86%). No cracks and gas pores were observed. In both layers, produced at various laser beam powers, the three zones were visible in the microstructure: laser-re-melted zone (MZ), heat-affected zone (HAZ) and the base material (alloyed material). The grains’ growth was observed in the HAZ of layer produced at lower laser beam
Fig. 15. Microhardness profiles of laser-alloyed layers with boron only [13] and laser-alloyed layers with boron and CaF2, produced at 1.56 kW (a) and 1.95 kW (b).
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Fig. 16. Results of wear tests of laser-alloyed layers with boron only [13] and laser-alloyed layer with boron and CaF2 at laser beam power of 1.56 kW (a, c) and 1.95 kW (b, d).
Fig. 17. The surface of specimens after wear test in the contrast of backscattered electrons; laser-alloyed Inconel®600-alloy with boron and CaF2 at laser beam power of 1.56 kW (a) and 1.95 kW (b).
power of 1.56 kW. In the HAZ of self-lubricating layer produced at 1.95 kW, the decrease in the size of grains was noticed. The recrystallization of grains in this zone during formation of subsequent laser tracks was the probable reason for such a situation. At the laser beam power of 1.56 kW, the temperature was too low for recrystallization. Of course, in the case of the layer, produced at the power of 1.96 kW, there was also grains’ growth, but the temperature increased as a consequence of the formation of subsequent laser tracks. Probably, this temperature was sufficient to cause recrystallization and enabled to obtain the smaller grains in the HAZ.
The microstructure of MZ consisted of nickel, chromium and iron borides (Ni3B, Ni2B, CrB, Cr2B, Fe3B) and Ni–Cr–Fe-matrix (re-melted substrate) as well as CaF2 particles. The microstructure of the self-lubricating layer depended on the laser beam power. The layer, produced at a higher laser beam power (1.95 kW) was characterized by smaller precipitates of the dark phase, identified as CrB and Cr2B borides, which were evenly distributed in the layer. The uneven distribution of the dark phases of various sizes was observed for the layer produced at 1.56 kW. In the re-melted zones, apart from irregular particles of chromium borides, the dendritic and globular precipitates of nickel,
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Fig. 18. Worn surfaces of laser-alloyed Inconel®600 with boron and CaF2. EDS patterns of calcium, chromium, nickel and iron.
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Fig. 19. The thickness of tribofilm; laser-alloyed Inconel 600-alloy with boron and CaF2 at laser beam power of 1.56 kW (a) and 1.95 kW (b).
Fig. 20. Scheme of tribofilm formation: scheme of the wear test (a), initial stage consisting in grinding-in (b,c), uncovering the lubricant particles and smearing the lubricant on the surface of specimen (d), formation of tribolfilm of diversified thickness (e).
wear intensity factors: 1.96 and 1.88 mg cm−2 h−1, for the layer produced at 1.56 kW and 1.95 kW, respectively. The advantageous influence of solid lubricants on the wear resistance was also confirmed by the measurements of relative mass loss (Δm/mi). The solid lubricant particles caused the formation of the tribofilm on the worn surfaces of laser-alloyed layers with boron and CaF2. The tribofilm was characterized by diversified thickness, obtaining up to approximately 3 µm. The presence of the tribofilm reduced wear of mating parts and improved their tribological properties. The mechanism of laser alloying with solid lubricants was developed (Fig. 21), showing the formation of re-melted and heat-affected zones, convection and vortex motions in molten pool and temperature distribution during laser treatment.
chromium and iron borides were visible among the soft Ni-phase as well as CaF2 particles, locating close to the surface. During laser alloying, these particles were partially melted and vaporized, partially melted and re-solidified. The self-lubricating layer, produced at a higher laser beam power, was characterized by a higher dilution ratio (0.56) and layer thickness (542 µm) compared to the layer produced at 1.56 kW (DR=0.4, tMZ=384 µm). The significantly lower hardness was characteristic of the self-lubricating layers in comparison with the laser-alloyed layers with boron only. In spite of that, the significant increase in wear resistance of laseralloyed layers with boron and CaF2 was observed. The self-lubricating layers were more resistant to wear, obtaining the lower values of mass
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Fig. 21. Scheme of the two-stage process of laser alloying of Inconel®600 with boron and CaF2.
Acknowledgements
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