May/June 2010 MPR 290026-0657/10 ©2010 Elsevier Ltd. All rights reserved.

special feature

‘Fine tuned’ steels point the way to a focused futureStrong materials that combine wear resistance with low coefficients of friction can be tailored to suit particular engineering applications.That’s just one example from materials scientists in Brazil that illustrates the possibilities for increasing efficiency, while scoring environmental points by cutting waste...

As technical development presses forward, functional high-per-formance materials need to be “fine tuned” for particular

applications. In many mechanical engineer-ing situations that means materials that have high strength and high wear resistance combined with a low friction coefficient.

Worldwide, a large proportion of the mechanical energy produced by machines is wasted by deficient lubrication, and converted into heat by friction. In more extreme examples, in addition to energy loss, the heat generated negatively affects the performance of the mechanical system. Conversely, a low friction coefficient between mechanical parts allows energy saving and increased durability of mechanical systems. It also contributes to environmental preservation by cutting waste. Dry self-lubricating bulk sintered materials may have a role to play in such situations.

They can be processed by powder metallurgy techniques and several compaction techniques are available for consolidating the powder mixtures into

mechanical parts of the desired shape. Although uniaxial die pressing is most economic, powder injection moulding (PIM) leads to nearly fully dense materials and has the advantage of enabling preform microstructural refinement. That is not feasible with the current uniaxial die cold pressing process.

Dry self lubricating parts have been used for several decades in household and light office equipment such as print-ers, electric shavers, drills, and blenders. Metallic matrixes of alloys based on cop-per, iron and nickel are the most com-monly used materials. Compounds such as MoS2, WS2, MoSe2, hBN, low melting point metals such as silver, tin and lead, as well as graphite are the most used solid lubricants [1, 2, 3, 4, 5]. Most of the dry self lubricating composites developed in the past contained a high percentage of solid lubricant particles (15 to 40 v/o). This was added to obtain a low friction coefficient, but resulted in a largely discon-tinuous metallic matrix that shows poor mechanical strength.

This work carried out by the research group at the Federal University of Santa Catarina was aimed at developing new dry self lubricating materials that have low friction coefficients combined with high mechanical and wear resistance. The combined high mechanical and tribological performance results from the simultaneous implementation of specific

requirements related to the mechanical properties of the metallic matrix and the microstructural parameters designed for the composite material. These features are basically the hardness and degree of continuity of the matrix; the nature and amount of solid lubricant; particle size of the dispersed solid lubricant particles, and the resulting mean free path between them.

There are two ways to disperse solid lubricant particles in the volume of a

The AuthorsThis article was drawn from Development of dry self lubricating sintered steels, with high mechani-cal and wear resistance combined with a low friction coefficient, a paper by Cristiano Binder1, Gisele Hammes1, Renan Schroeder1, Aloisio N. Klein1, José Daniel B. de Mello2, Roberto Binder3, Waldyr Ristow Junior4.1LABMAT- Engenharia Mecânica-UFSC, Florianópolis, SC- Brazile-mail: ctbinder@emc.ufsc.br2LTM- Engenharia Mecânica-UFU, Uberlândia, MG- Brazil3Wirhlpool- EMBRACO, Joinville, SC- Brazil4CPDL- Steelinject, Caxias do Sul, RS- Brazil

More to comeA second feature in this series of Brazilian research articles will appear in the September/October edition of Metal Powder Report.

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metallic matrix in order to get a dry self lubricating sintered composite [6, 7]: 1. Blending the powder of solid lubricant

with powders of the metal matrix alloy by a simple mixing process; and

2. Generating particles of the solid lubricant phase in situ during sintering by dissociation of a precursor mixed to the powders of the metallic matrix.

In cases where the composite is obtained by mixing the metallic matrix powders with the solid lubricant particles, the shear stresses that occur during mixing and compact-ing spread the solid lubricant by shearing between the powder particles of the metal matrix. This leads to an undesirable arrange-ment in which the solid lubricant covers the metallic particles to a large extent [8]. Insoluble solid lubricant layers covering the metallic particles, that form the metallic matrix by sintering, hinders the formation of sinter contacts between them. This results in a metallic matrix with a high degree of dis-continuity and leads to a composite material with reduced mechanical strength.

In order to overcome these limita-tions, the researchers used a second way to obtain a homogeneous dispersion of discrete particles of solid lubricant in the volume of the metallic matrix. This is achieved by in situ formation of graphite nodules in the volume of the sintered steels during sintering by dissociation of silicon carbide (SiC) particles mixed with the metallic matrix powders prior to the PM shaping step [6, 7].

The research demonstrates and dis-cusses some results of a study aimed at the development of new dry self lubricating materials that have a low friction coeffi-cient combined with high mechanical and wear resistance. The concomitant high

mechanical and tribological performance results from the simultaneous implemen-tation of specific requirements that can be grouped into two sets:a) Optimisation of the microstructure of

the composite to obtain a high degree of continuity of the metal matrix, which is the continuous phase that gives strength to the composite; and

b) Mechanical properties (hardness, strength and toughness) of metal matrix selected for the dry self-lubri-cating composite material

Microstructure optimisation

The following aspects have to be consid-ered for microstructural optimisation of the composite:a) Content of solid lubricant (v/o);

b) Size and size distribution of solid lubri-cant particles; and

c) Mean free path between the solid lubri-cant particles.

The solid lubricant dispersed in the volume of the composite material should be a com-pletely discontinuous phase, ie, like discrete particles uniformly dispersed in the metallic matrix. Certainly, there is an optimal size of the dispersed solid lubricant particles as well as an ideal mean free path between them that makes it possible to obtain a completely continuous metallic matrix. Figure 1 shows, schematically, the ideal situ-ation. Theoretically, a regular distribution of solid lubricant particles is desired so that each particle has to provide lubricant for a well defined area of the interface, as shown schematically in Figure 1. That means that a uniform distribution of mono-sized solid lubricant particles should be sought.

Such an ideal distribution of the solid lubricant particles is not obtained simply by blending metallic and solid lubricant powders. The shear stresses that occur during the PM processing of the compo-nents spread the solid lubricant between the metal powder particles by shearing, resulting in layers instead of discrete par-ticles. The presence of a layer of insolu-ble solid lubricant between the metallic particles hinders the formation of metal-metal contacts during sintering and leads to a high degree of discontinuity in the metallic matrix of the composite material. Consequently, the strength of the compos-ite material decreases.

Figure 1. Desirable distribution of the lubricant particles in the volume of the matrix and the corresponding area to be lubricated by each particle (schematically).

Figure 2. Hybrid sintering furnace (or plasma DC reactor) designed for plasma assisted debinding and sintering of powder injection moulded parts [11].

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To overcome these difficulties, two possibilities seem to be useful. The first one is to rearrange the solid lubricant phase in discrete agglomerates by the cap-illary action of a liquid phase. The liquid phase spreads and penetrates the interfaces between the metal and the lubricant, thus pushing the solid lubricant and rearrang-ing it in agglomerates. The second is to produce the solid lubricant phase in situ during sintering by decomposition or dis-sociation of a precursor phase. In this case, the precursor may have high enough shear strength to permit its effective dis-persion among the metal matrix powders.

To further minimise the interruption of the metallic matrix of the composite material, the amount (v/o) of the dispersed solid lubricant must be kept as low as pos-sible. It seems to be possible to achieve low friction coefficients even with a low amount of solid lubricant in a composite in which all the microstructural param-eters are optimised in terms of tribological knowledge [6]. Of course, the performance depends also on the particular nature of solid lubricant used.

The metallic matrix of the composite must be hard and tough enough to avoid the occurrence of micro-plastic deforma-tion by friction and wear. The mass flow that occurs by plastic deformation gradu-ally covers the particles of solid lubricant

and blocks the self-replenishment or resupply of lubricant to the interface. This means lubrication will be poor and the friction coefficient will increase at the interface of surfaces in relative motion.

The mechanisms involved in friction and wear are dependent on several specific parameters, such as load and effective con-tact area, surface roughness, hardness and the strength of the material. The greater the hardness of the material, the more the plastic deformation is reduced. Further, it is important that the metallic matrix doesn’t chemically react with the solid lubricant phase.

The study called for sintered steels with several chemical compositions containing variable contents of the alloying elements Si, Ni and Mo to be processed by powder

injection moulding. The iron powder used was the Carbonyl BASF CL-OM, with a mean particle size of 7.8 μm. This carbo-nyl iron powder contains residual levels of carbon (0.8 w/o) and oxygen (about 0.2 w/o) that react with each other during sintering, forming CO, so that the result-ing carbon is about 0.6 mass percent. The alloying elements Ni and Mo were mixed into the feedstock in the form of elemental Ni and elemental Mo powder. The Si was added in the form of silicon carbide (SiC) powder aiming at its dissociation during sintering. As a consequence, graphite nod-ules dispersed in the volume of the sintered steels were obtained. Increasing contents of SiC powder (1, 2, 3, 4 and 5 w/o SiC) with mean particle size of 10 μm were added to the mixture.

The feedstock preparation for powder injection moulding was carried out in a Haake Sigma mixer (180°C, 70 rpm, 90 min) using 8 w/o of an organic binder system containing paraffin wax, polypro-pylene, stearic acid (surfactant), ethylene vinyl acetate copolymer (EVA) and amide wax. The samples for tensile test, hardness and tribological test were injected with a pressure of 100 MPa using an Arburg 320S injection moulding machine. The debind-ing was carried out in two steps: chemi-cal debinding followed by plasma assisted thermal debinding. The chemical step con-sisted of dissolution of the low molecular weight components of the binder system in hexane, forming an open pore network within the injected parts. Thermal debind-ing and sintering were performed in a single thermal cycle in a plasma reactor, using the Plasma Assisted Debinding and Sintering (PADS) process developed in [10].

The equipment (plasma reactor/fur-nace) designed for the PADS process [11, 12], shown in Figure 2, allows the control of processing temperatures and heating

Figure 3. Microstructure of the sintered steels: a) Fe + 0.6 C + 4 Ni; b) Fe + 0.6 C + 4 Ni + 2 SiC (w/o).

Figure 4. Microstructure of the steels sintered for 1h at 12000C: a) Fe + 0.6 C + 2 SiC; b) Fe + 0.6 C + 4 Ni + 2 SiC (w/o).

Figure 5. Chemical composition line profile showing the concentration of silicon and carbon along the graphite nodule and the surrounding metal matrix. Composition of the analysed steel: Fe + 0.6 C + 2 SiC (w/o).

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rates independently of the plasma param-eters since it has (in the same processing environment – vacuum chamber), elec-trodes for the generation of plasma DC and electrical heaters for heating control. The samples were set on ceramic plates sup-ported by the structure of the anode and were, initially, processed in floating potential plasma. The abnormal hydrogen glow dis-charge was generated at a pressure of 133 Pa (1 Torr) and the gas flow was adjusted to 3.33×10−6 m3 s−1 (200 sccm). The power supply voltage biased to the cathode was fixed at 500 V. A heating rate of 0.7°C/min was fixed up to 500°C for the plasma assisted thermal debinding step. After debinding was completed, the heating rate was increased to 5°C/min up to the sintering temperature. The isothermal sintering was performed at several temperatures in the range from 1110°C to 1200°C, over 60 minutes.

After sintering, the samples were char-acterised via scanning electron microscopy (SEM) and Raman spectroscopy. A semi-quantitative analysis of the chemical com-position was carried out via energy disper-sive X-ray microanalysis (EDX) apparatus coupled to the scanning electron micro-scope. Tensile tests were made using MTS (model 609) equipment. The tribological characterisation was performed in air in an un-lubricated reciprocating wear test, in which both contact potential and friction force were continuously recorded (acqui-sition rate of 1 Hz). A hard steel sphere was held on a pivoted arm and rested against the specimen surface under con-stant stroke (6 mm) and frequency (2 Hz). The tests were conducted under controlled relative humidity (50%) and temperature (22 ± 4°C) conditions. By increasing the normal dead weight load in increments of 7 N at an interval of every 10 minutes the surface durability was determined. In this test, the surface durability was defined as

the work (N.m) at which the value of the friction coefficient first rose above 0.20 (lubricity effect). The friction coefficient was calculated in this lubricious regime. The counter-body was a 5 mm diameter AISI 52100 hard steel ball. The ball surface was used in the as-received condition and a new surface region was used for each test. In each case, the results reported are the average from at least four tests under identical experimental conditions.

Figure 3 shows examples of the micro-structures of the materials after sintering for 1 hour at 1200°C, as observed in optical microscopy. While the feedstock without SiC resulted in a matrix formed by perlite and ferrite, when SiC powder was added to the feedstock, graphite nodules were formed in the matrix volume during sintering. They were surrounded by ferrite rings, as can be seen in Figure 3b [6, 7, 9]. The formation of a microstructure like this was expected from thermodynamic knowledge because its formation results from the reactions that occur during sintering between the different phases mixed into the feedstock. The extent and the amount of formed ferritic rings

depend on the content of silicon carbide added to the feedstock and on the sintering parameters. During sintering of samples that contained SiC particles, at the initial stage of SiC dissociation, the Si atoms and the carbon atoms generated were both dissolved in the ferrous matrix. However, the continuous enrichment of the ferrous matrix with Si around the former silicon carbide particles lead to the stabilisation of the body centered cubic structure of the fer-rous matrix (ferrite), in which the solubility for the carbon atoms was very low (solubil-ity ≤ 0.022 w/o) [13], drastically reducing the dissolution of graphite. The dissolution of silicon, in contrast, was maintained. As a consequence, ferrite rings are formed around the former SiC particles. In this way the carbon was retained and forms graphite nodules, as shown in Figure 3b.

The evolution of the microstructure depends on the sintering parameters (tem-perature, time and atmosphere) and on the previous composition of the ferrous matrix, ie, the presence of other alloying elements in the matrix. For example, the presence of carbon in the matrix prior to the start up of SiC dissociation reduces the dissolution of the carbon originated from the dissocia-tion of SiC, and increases the size and the amount of the graphite nodules formed. The presence of Ni, which has a strong tendency to stabilise the γ-phase of the iron matrix (austenite stabilisation), influences positive-ly the formation of the graphite nodules, rounding their shape and restricting their width. This can be observed by comparing the microstructures shown in Figure 4. While Figure 4a shows the microstructure of a sam-ple without Ni, Figure 4b shows the micro-structure of a sample containing 4%Ni.

Figure 6. Structure of graphite nodules of the sintered steel with composition: Fe + 0.6 C + 4 Ni + 2 SiC (w/o) (sintered for 1h at 1200°C): a) FEG-SEM image: graphite nodule (taken on a fractured surface); b) FEG-SEM image: Graphite foils inside the nodules.

Fric

tio

n c

oef

ficie

nt

Du

rab

ility

(N.m

)

SiC content (w/o)

Friction coefficient Durability

Figure 7. Friction coefficient as a function of the content of SiC added to the feedstock, measured on alloys with composition Fe + 0.6 C + variable SiC content, sintered for 1h at 1150°C.

34 MPR May/June 2010 metal-powder.net

As can be seen, with the presence of Ni, which is a γ-phase stabiliser, the graphite nodules appear to be larger and have a better shape factor, (more rounded).

Figure 5 shows the chemical composi-tion profile along a line crossing a graph-ite nodule, taken by microprobe analysis (EDS-SEM). By this, the presence of high carbon content inside the nodule and a higher content of silicon in the matrix out-side the nodule, along of the ferritic ring, could be confirmed.

The SEM photographs presented in Figure 6 show the structure inside the graphite nodules. The micrographs were taken on a sample prepared simply by frac-ture, since the conventional metallographic preparation of the samples smears out the surface of the graphite nodules. As it can be observed, the remaining carbon forms graphite nodules which show a structure similar to a pile of graphite layers with thickness in the nanosize range.

Aiming to study the tribological and mechanical properties of the dry self lubri-cating sintered steels, several sets of sam-ples varying the chemical composition by addition of increasing SiC contents and specific alloying elements (C, Ni, Mo and Si) were produced by metal injection moulding. The samples were sintered for one hour at several isothermal sintering temperatures, taken in the range from 1100 to 1200°C, in the PADS furnace. Figure 7 presents some results (friction coefficient and its durability) of a set of samples with the composition Fe + 0.6 C + X SiC (X = 1, 2, 3, 4 and 5 w/o SiC), sintered for 1 hour at 1150°C. As expected, increasing the amount of SiC added to the feedstock

leads to the formation of more graphite nodules and consequently drops the fric-tion coefficient and increases its durability.

As can be seen, it is not necessary to introduce large amounts of solid lubricant in order to drop the friction coefficient down to desirable values (for solid lubrica-tion defined as μ ≤ 0.2 [14]). However, higher durability of the low friction coefficient is obtained by increasing the SiC content that generates larger amounts of graphite by dis-sociation of SiC during sintering. Figure 8 presents the corresponding tensile strength, hardness and elongation of the same group of samples. As mentioned in the section methodology development, the solid lubri-cant does not contribute to the mechanical strength of the dry self lubricating compos-ite. The lower the content of solid lubricant added to the feedstock, the greater the degree of continuity of the metal matrix and, con-sequently, higher is the mechanical strength

of the composite. The results of hardness and strength as a function of SiC content in Figure 8 show two concurrent effects: solid solution hardening of the matrix by the dis-solution of silicon that comes from the dis-sociation of SiC and weakening of the com-posite matrix due to the increasing amount of graphite nodules formed with increasing SiC content added to the feedstock.

Figure 9 presents the tensile strength, yield strength, hardness and corresponding friction coefficient of the steels, sintered for one hour at 1150°C, in which the alloying elements Ni (4 w/o) and Mo (1 w/o) were added to the Fe + 0.6 C + 3 SiC (w/o) alloy. As expected, the mechanical properties (strength and hardness) of these sintered steels were improved by the addition of the alloying elements. The high strength and hardness of the sintered steels containing Ni and Mo is due to the martensitic microstructure formed after sintering. The hardenability is high enough to form martensite by the normal cooling rates used in this work, after sintering the samples. Note also that the friction coefficient decreases with increasing hardness and strength of the material. Similarly, the durability of the friction coefficient under wear test is positively affected by the mechanical properties of the dry self lubricating sintered steel, as can be seen in the Figure 10.

As main conclusion of the study, it can be safely stated that it is possible to design dry self-lubricating sintered steel microstructures specifically for each particular engineering application where low friction and high strengths are required. The actual values of these properties depended on the chemi-cal composition of the matrix considering the alloying elements added to increase the

Figure 8. Tensile strength, hardness and elongation measured on the sintered Fe + 0.6 C + variable content of SiC (w/o).

Figure 9. Tensile strength, hardness and elongation measured on the sintered steels.

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References[1] A. Erdemir, “Review of engineered

tribological interfaces for improved boundary lubrication. Tribology International 38 (2005) 249–256;

[2] C. Donnet, A. Erdemir, “Historical developments and new trends in tribological and solid lubricant coatings”, Surface and Coatings Technology 180 –181 (2004) 76–84;

[3] Y. Pauleau, F. Thièry, “Deposition and characterization of nanostruc-tured metalycarbon composite films”, Surface and Coatings Technology 180 –181 (2004) 313–322;

[4] H. Kato, M. Takama, Y. Iwai, K. Washida, Y. Sasaki, “Wear and mechanical properties of sintered copper–tin composites containing graphite or molybdenum disulfide”, Wear 255 (2003) 573–578;

[5] X. Dangsheng, “Lubrication behav-ior of Ni-Cr-based alloys containing MoS2 at high temperature”, Wear 251 (2001) 1094–1099;

[6] A.N. Klein, C. Binder, G. Hammes; J.D.B. de Mello, W. Ristow Jr, R. Binder, Self lubricating sintered

steels with high mechanical resis-tance obtained via in situ formation of solid lubricant particles during sintering. Proceedings of EURO PM2009, Vol. 1(2009) 191-196;

[7] R. Binder, A.N. Klein, C. Binder, G. Hammes, M.L. Parucker, W. Ristow Jr., “Composição metalúrgica de materiais particulados, produto sin-terizado autolubrificante e processo de obtenção de produtos sinterizados autolubrificantes”. Patent application PI 0803956-9, INPI, Brazil, 2008;

[8] R. Binder, A.N. Klein, C. Binder, W. Ristow Jr., “Composition of particulate materials for forming self-lubricating products in sintered steel, product in self-lubricating sin-tered steel and process for obtain-ing self-lubricating products in sin-tered steel”. Patent application PI 0805606-4, INPI, Brazil, 2008;

[9] C. Binder, “Desenvolvimento de novos tipos de aços sinterizados autolubrificantes a seco com elevada resistência mecânica aliada a baixo coeficiente de atrito via moldagem de pós por injeção”. PhD Thesis, PGMAT, UFSC, Brazil, 2009;

[10] A.N. Klein, J.L.R. Muzart, A.R. Souza, M.C. Fredel, P.A.P. Wendhausen, R.M. Nascimento, “Process for removal of binders from parts produced by powder injection molding”, US patent ( US 6,579,493 B1), 2003;

[11] R. Machado, J.L.R Muzart, A.N. Klein, W. Ristow Jr., M.C. Fredel, P.A.P. Wendhausen, D. Fusão, L. Mendes, “Industrial plasma reactor for plasma assisted thermal debind-ing of powder injection molded parts”, PCT (WO 2006012718 ) and INPI (PI-0403536-4), 2004;

[12] P.A.P. Wendhausen, D. Fusão, A.N. Klein, J.L.R. Muzart, W. Ristow Jr, R. Machado, “Plasma Assisted Debinding and Sintering: Process and Equipment. In: Proceeding of the Powder Metallurgy World Congress & Exhibition 2004, EURO PM2004, Vienna, 2004. v. 4. p. 137-142;

[13] W.D. Callister Jr., “Fundamentals of Materials Science and Engineering”, John Wiley & Sons, Inc., 2001, p. 303;

[14] K. Miyoshi, Solid Lubrication, Fundamentals and Applications, Marcel Dekker, Inc, 2001;

strength of the steel, the contents of SiC added to the feedstock and the processing parameters. Based on the results reported here, it can be concluded specifically that:

The dissociation of SiC compound leads to the formation of graphite nodules in a ferrous matrix as a result of the ferrite stabilisation effect of the chemical element Si. Thus, the distribution of

solid lubricant nodules in the matrix is defined by the mixing process during the feedstock preparation. In this way, one can bypass the effect of spreading the solid lubricant in continuous layers between the powder particles of the metallic matrix during the steps of mixing and shaping of dry self lubricating materials.The graphite nodules formed by dis-

sociation of SiC have a rounded shape and the carbon crystallizes in graph-ite foils with thickness in the nanosize range. Such graphite structure seems to be more appropriate for lubrication. The friction coefficient decreases with the amount of formed graphite to values like 0.06, enabling to obtain sintered steels that combine high mechanical strength with low friction coefficient. For a constant amount of graphite nod-ules, the friction coefficient decreases with increasing hardness and strength of the metallic matrix of the dry self lubricating sintered steel. The durability of the friction coeffi-cient raises with increasing hardness and strength of the metallic matrix of the dry self lubricating sintered steel

AcknowledgementsThe authors acknowledge the Brazilian founding agencies CNPq and FINEP, as well as Whirlpool/Embraco and Lupatech/Steelinject for funding this research program and technical contributions.

Figure 10. Friction coefficient and the durability of the solid state lubrication conditions for two different alloys sintered for 1 hour at 1150°C.