ADVANCED INSTRUMENTAL TECHNIQUES IN TRIBOTECHNICAL DIAGNOSTICS

ПЕРЕДОВЫЕ АППАРАТНЫЕ МЕТОДЫ В ТРИБОТЕХНИЧЕСКОЙ ДИАГНОСТИКЕ

Assoc. Prof. Dr. Machalíková J.1, Assoc. Prof. Dr. Chýlková J.2, Ing. Šelešovská R.2, PhD.

Jan Perner Transport Faculty 1, Faculty of Chemical Technology 2 – University of Pardubice, Czech Republic

Abstract: The paper deals with the utilization possibilities of choice instrumental techniques—Fourier transformation infrared spectroscopy, analytical ferrography and voltammetric methods in analysis of oils taken from engines and gear boxes of railroad and road vehicles. The results of a study of degradation of mentioned oils are presented. Voltammetric measurements were focused on determination of nitro compounds.. KEYWORDS: TRIBOTECHNICAL DIAGNOSTICS, ADVANCED INSTRUMENTAL TECHNIQUES, OIL ANALYSIS, VOLTAMMETRY

1. Introduction

Tribotechnical diagnostics (TTD) is a part of technical diagnostics, which are a set of non-destructive, non-demounting methods and instruments used for investigation of technical equipment state. Technical diagnostics (TD) take advantage of defects attributes; they mean changes of detectable output parameters of the diagnosed object. TTD methods use oils as a source of information about processes, changes and wear mode, which proceed in the technical systems. TTD resolve two problem areas:

1) establishing of the state of oil, extending its usability and forecasting of its degradation process.

2) establishing of mode, place and trend of mechanical system wear through the foreign substances incidence evaluation.

The degradation of oils is especially a result of its reactions with air oxygen. Range and rate of damage depend on many factors, e.g. temperature and the chemical composition of oil (including additives). The observation of chemical and physical changes provides correct ideas about the actual state of oil and about the possibilities of its next usage in working.

History and extent of lubricated component wear can observe various methods, which are suitable for

- determination of concentration of wear metals (e.g. OES-ICP, voltammetry),

- description of morphology and distribution of particles from metal abrasion, fibres from filter materials or contaminants from outside background (ferrography with consecutive visual analysis).

From the viewpoint of reliability, combustion engine is the most important element for majority of vehicles. The possibilities of usage of vehicles depend significantly on economic efficiency and reliability of the combustion engine. Therefore, special emphasis is put on the diagnostics of those elements, technical state of which influences the engine load, fuel and lubricant consumption. The tightness of the combustion chamber is one of the most important criteria of the technical state of an engine (especially the wear of cylinders, pistons and piston rings). The tightness of the combustion chamber influences directly the combustion process, i.e. load and other parameters. With the tightness decreasing, consumption of oils and fuel increases and engine cranking gets worse etc. Most of the methods established so far that were used for detection of tightness of the combustion chamber do not allow performing the control without partial demounting. Measuring the compression pressure, measuring the tightness of the combustion chamber using compressed air or measuring the gas penetration to the crank case are examples of direct methods for observation of tightness of the combustion chamber. Detection and determination of nitro compounds in motor oils belong to indirect (non-demounting) methods. It is possible to use for example FTIR spectroscopy

(Černý et al. in paper [1]) or electrochemical methods, especially voltammetry.

2. Instrumental methods in tribotechnical

diagnostics − experiments, results and discussion

2.1. Infrared spectroscopy

This analytical method is suitable especially for identification and structural characterization of organic compounds. It is based on measurement of the absorbance of infrared radiation by the material analysed. The analysis itself was carried out using FTIR spectrometer Vector 22 (Bruker) in the spectral range of wave length 600–4000 cm-1, with the differentiation of 4 cm-1 and with the scan number 32, using the ATR technique.

The observation of the absorbance changes in infrared spectrums can serve for monitoring of oils degradation, its contamination and decrease of additives (see Fig.1): losses of antiwear and antioxidant additives (ZnDDP) vent it by absorbance decreasing in the area of 1050–950 cm-1. When the decrease is under 20 % of original concentration of ZnDDP, it is recommended for an exchange of the oil filling. Similarly, it is possible to monitor the decrease of antiwear additive TCP − it is following by absorbance decreasing in the range of 990–960 cm-1. The oxidation of hydrocarbons in oils evokes increasing of absorbance in the area along 1710 cm-1. The contamination of oils by combustion products can be observed along 1900 cm-1, the increase of absorbance between 1650–1580 cm-1 reflects the presence of organic nitro compounds from blowing-by of combustion gases from the combustion chamber; these effects are indicators of wear of the piston group. The presence of water is seen in a spectrum as a wide band in the range of 3600–3300 cm-1 and the intrusion of fuel to the oil as the increase of absorbance in the area of 815–800 (or 750) cm-1. Reviews of typical groups and compounds characteristic for motor and gear oils are presented in papers [2–4].

The authors of this paper have been dealing with the FTIR spectroscopy of vehicle oils for five years. During this time, more than 700 samples of new or wear motor and gear oils were analysed. The process of service wear was observed in oils sampled from

- vehicles of Czech Railways (locomotives of the 730, 732, 751 series, motor vehicle of the 810 series) in collaboration with DKV Česká Třebová,

- buses (Karosa, Vysoké Mýto) in collaboration with ČSAD České Budějovice, DZ Písek, DP ČAS Znojmo, branch Moravský Krumlov and ČSAD Pardubice,

- motorcycle KTM 250 SX type 2003,

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- personal motor cars Škoda Fabia 2,0 1/85 kW and 1,9 TDI/74 kW,

- heavy goods vehicles (trailer-towing vehicles SCANIA R 124 LA4x2NA 380 with mechanical 12-stage gear, MAN 19.422 of the F 2000 series, RENAULT Magnum E-Tech),

- tractors ZETOR 161 45, 101 11, 8111 and 6211 and agricultural machines.

The information about new oils is filled into the database, in which the FTIR spectrums themselves and available catalogue data deals with the individual oils are archived. The database was created in the computer environment of MS Access XP. Each product (mostly oils, recently also plastic lubricants) is associated with different information—producer name and its Internet address, oil name, oil usage and description, SAE viscosity classification, performance specification (ACEA, API, specification of vehicle producers), HTML reference for measured FTIR spectrums in original date files, some other parameters of oils (kinematic/dynamic viscosity at different temperatures, viscosity index, density, kindling temperature, pour point etc.). With respect to the character of the workplace, the above mentioned results are used besides the education.

2.2 Ferrography

Ferrography is a method of tribodiagnostics based on separation of extraneous substances which are present in the oil filling of lubricant systems, from the oil matrix itself. It uses the sedimentation of particles on a special sheet besides the flow of the oil sample in a strong non-homogeneous magnetic field. At present, two versions of ferrography are applied: direct reading ferrography and also analytical ferrography. Morphology, colour, surface character and other visual characteristics of sediment particles which carry important information about dominant type of attrition and machine component wear are observed using bichromatic microscope.

Fig. 1: Decrease of additives in consequence of SAE 15W-40 motor oil deterioration followed by a decrease of absorbance in the range

of 1050–950 cm-1. (1 – new oil, 2 – used oil before exchange; A − absorbance, f − wavenumber [cm-1])

In the framework of complex analysis of motor and gear oils,

ferrographic evaluation was performed for choice samples. Ferrograph Reo 1 (ReoTrade, Ostrava) in connection with the bichromatic trinocular microscope H 6000 (Intraco Micro, Tachlovice) and digital camera Nikon Coolpix 4500 was used for these measurements.

Based on a study of particles found on ferrograms, it is possible to say that in most investigated cases, wear of motor components was progressive with normal development. The incidence of non-metal particles, especially fibres from filters, was comparatively frequent (see Fig. 2A). The particles frictional by combination of rolling and sliding friction were found in gear oils occasionally (they form besides rolling of cog wheel and so on). As a consequence of high contact pressures on the cogs surface and

high rates, microwelds are created and characteristic particles with deep striated surface are formed by ripping out of welded micro bridges (see Fig. 2B).

A

B

A

B

Fig. 2: Ferrogram obtained from the Motorex EP 80W gear oil

(running-in of motorcycle KTM 250 SX type 2003 gear) 2A – chains of ferromagnetic abrasion and large lamellar

particles, magnification 100× 2B – typical particle formed by ripping out of welded micro

bridges, magnification 800×

Quantitative evaluation of obtained ferrograms is carried out using a densimeter which measures direct optical density of a ferrogram. The second method is based on digital image processing of a ferrogram (it can evaluate the ratio of the area of a ferrogram covered by particles, calculate formative factors of the individual particles etc.).

2.3 Voltammetric determination of nitro compounds

Voltammetric determination of nitro compounds is based on the electrochemical reduction of the –NO2 group. This nitro group can be reduced to a –NO, –NHON or –NH2 group and it depends on the base electrolyte used [5]. The aliphatic mononitro compounds are reduced to the appropriate alkyl hydroxylamine in an acidic medium. The position of the peak corresponding with this reduction is independent on the rest of molecule. The aromatic mononitro compounds (nitrobenzene is its simplest representative) are reduced to N-phenyl hydroxylamin in water or a water-alcoholic medium at all pH values. The peak position of this reduction depends on the pH value, ionic strength and on the presence of surface active substances. Substituted nitrobenzenes are reduced by the same reaction mechanism (it is four electron reduction) but the peak position is different. The aromatic polynitro compounds (e.g. m-dinitrobenzene) yield one peak at the electrochemical reduction in an acidic medium which corresponds to the reduction of both nitro compounds. In a neutral or alkaline medium, the individual nitro groups are reduced by a degree and two very well developed peaks are seen on the obtained voltammogram. The examples of choice aliphatic nitro compound half-wave potentials in dependence on the pH values of the base electrolyte are shown in Tab. I, and the half-wave potentials of choice aromatic nitro compounds are summarized in Tab. II. Based

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on the above mentioned facts, it is possible to conclude that nitro compounds are reduced in a certain range of negative potentials in dependence on the pH value of the base electrolyte.

Using voltammetry for determination of nitro compounds in oils, the separation of analysed compounds from a complicated matrix and the quantitative evaluation of obtained curves are the greatest problems. The electrochemical analyser EP 100 VA (HSC service, Bratislava) served for voltammetric measurements. Hanging mercury drop electrode (HMDE) served as a working electrode (Polaro-Sansors, Praha), silver chloride electrode (of the RAE 113 type) as a reference and platinum wire as an auxiliary electrode (both Monokrystaly, Turnov). Clear oil M7 ADS III was used as the reference oil and all model mixtures were prepared from it. The unknown samples were taken from combustion engines of various road and rail vehicles before the end of their oil exchange period. The nitro compound voltammetric analysis in oils was tested using the following standards: nitromethan, 2-nitropropan, nitrobenzene, 2-nitrotoluene, p-nitrotoluene, o-nitrophenol, p-nitrophenol and 2,4-dinitrophenol. Ammonium buffer (pH 10) served as a base electrolyte and all measurements were carried out in presence of 76% of ethanol. The examples of measured voltammetric curves of nitrotoluene and its dependence on the concentration of analyte in the sample are shown in Figure 3. It is obvious from this figure that the peak increases linearly.

Tab. I: Half-wave potentials of choice aliphatic nitro compounds in

dependence on the pH value of the base electrolyte

Tab. II: Half-wave potentials of choice aromatic nitro compounds in dependence on the pH value of the base electrolyte

Nitro

compound pH Half-wave

potential [mV] 2.5 -0.34 4.1 -0.46 7.4 -0.62

nitrobenzene

9.2 -0.74

2.5 -0.16, -0.37 4.1 -0.25, -0.54 7.4 -0.35, -0.73

p-dinitrobenzene

9.2 -0.39, -0.84

2.5 -0.34, -0.55 4.1 -0.52, -1.0 7.4 -0.62, -1.1

p-nitrotoluene

9.2 -0.73

2.5 -0.2, -0.41 4.1 -0.32, -0.57 7.4 -0.4, -0.73

2,3-dinitrotoluene

9.2 -0.46, -0.83

2.0 -0.27 4.0 -0.41 6.0 -0.54 8.0 -0.69

o-nitrophenol

10.0 -0.8

Developed procedure of oil pre-treatment before voltammetric measurements 0.5–2 g of oil (according to the nitro compounds incidence) is put into a 25 ml Erlenmeyer bottle. 10 ml of 96% ethanol is added to the sample and the mixture is inserted to the ultrasonic bath for 5 minutes where the accomplished contact between both immiscible phases is carried out and nitro compounds come to the ethanolic solution. After taking out the bottle from the bath, 2 ml of ammonium buffer (2 mol.l-1) are added to the mixture and after 5 minutes the mixture is filtered through the glassy wadding stock. Pure ethanolic solution is then used for voltammetric analyses.

The developed procedure of nitro compound isolation from oils was tested using model samples prepared from the clear M7 ADS III oil for the first time. Known amount (46.7 μg) of nitrotoluene was added to this oil; its weight was in the range of 1.4–1.8 g. Determined values of nitrotoluene are summarized in Tab. III. It is obvious from the table that determination errors achieve rather negative values and range between -2.7 and -10.24 relative percent. It is acceptable in the area of operative analysis. It can be concluded that the described isolation process with following voltammetric analyses is applicable for unknown oil samples which may contain products of nitration.

Fig. 3: Voltammetric curves of the o-nitrotoluene reduction in dependence on concentration

Method – DPV, electrolyte – 0,2M ammonium buffer (pH 10) + 76% ethanol, concentration of o-nitrotoluene: 20–160 μg/10 ml.

Tab. III: Reproducibility and accuracy of nitrotoluene determination in the M7 ADS III oil

Number of analyses

Weight of oil* [g]

Determined value

[μg]

Relative error [%]

1 1.56 45.44 -2.7 2 1.44 44.77 -4.13 3 1.74 41.92 -10.24 4 1.75 44.77 -4.13 5 1.5 44.82 -4.03

* 46.7 μg of nitrobenzene was added to the oil weigh

Average: 44.34 μg Confidence interval (95 %): 42.63 – 46.06 μg Standard deviation: 1.38 μg 30 real samples of motor mineral oils (two of them were clear

unused oils) were analysed using the above mentioned electrochemical method. Nitro compounds were not detected in three samples, namely in two unused and one used oil. The other samples provided one or two well developed peaks in the potential range where nitro compounds are reduced. It is possible to deduce from this fact that nitro compounds are present in these samples.

Nitro compound

pH Half-wave potential [mV]

1.8 -0.75 4.6 -0.85 8.0 -0.9

nitromethan

11.6 -0.9 1.8 -0.73 4.6 -0.83 6.8 -0.89

nitroethan

11.6 -0.93 1.8 -0.71 5.1 -0.81 8.0 -0.98

2-nitropropan

10.9 -1.02

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The peak positions were a little different; this reflects creation of various products of nitration in a complicated oil matrix. Moreover, the independent infrared spectrum measurements confirmed the presence of nitro compounds in analysed oils. It was proven from the absorption band at the wave length of 1600 cm-1. Furthermore, clear unused oils contain a little amount of nitro compounds (probably from some additives); this is obvious from spectral measurements. Voltammetric measurements did not find this original nitro compounds. They can be caused by their smaller solubility in an ethanolic solution.

References

1. Černý, J et al. (2004) Nitrace motorových olejů a možnosti její kvantifikace. Proc. Conf. REOTRIB 2004. VŠCHT Praha, Czech Republic. Pp. 167-174. ISBN 80-7080-537-4.

2. Smith, G.C., Bell, J.C. (1999) Multi-technique surface analytical studies of automotive anti-wear films. Applied Surface Science. Vol. 144-145 (April), pp. 222-227.

3. Wurzbach, R. N. Airing Out Lubricant Oxidation. [cit. 2001-30-09] <http://www.noria.com/learning_center/category_article.asp?articleid=14&relatedbookgroup=Lubrication>.

Quantitative determination of detected nitro compounds is difficult with respect to the problem of a suitable standard finding. The result of analyses of an oil sample (from a locomotive of the 754 series, ČKD K12V210DR engine, Mogul Diesel DT) is documented at Fig. 4. Curve 1 demonstrate the analyses of oil ethanolic extract, curves 2 and 3 illustrate the analysis of the same sample with the addition of 5 μl and 10 μl of a nitrotoluene standard solution. The peak position of samples with the standard addition reproduces the position peak of the sample very well. The amount of nitro compounds in this sample was evaluated using this measurements and the result was recalculated to the amount of –NO2 groups. Calculated value was 0.002 g/100 g of oil.

4. Finch, J. Using Oil Analysis to Monitor the Depletion of Defoamant Additives. [cit. 2001-29-09]. <http://www.noria.com/learningcenter/categoryarticles.asp?articleid=103&relatedbookgroup=Lubrication>.

5. Barek, J., Švagrová I. (1990) Polarografické a voltametrické stanovení nitrosloučenin. Chemické Listy. Vol. 84, pp.1042 – 1061.

Two different ways of the evaluation of nitro compounds amount in oils are possible: the first is the above mentioned procedure using the standard of nitro compound which yields its reduction peak at the same potential as the unknown determined compound; the second procedure is using only one representative standard for all nitro compounds aside from the peak positions. Both proposed possibilities will be subject to further research.

Fig. 4: Voltammetric determination of nitro compound

in the Mogul Diesel DT oil using the method of standard addition.

3. Conclusions Tribotechnical diagnostics is based on the individual checking of the oil degradation degree in every machine equipment. It utilizes the lubricant as an information source for the processes and changes in technical systems, in which lubricants are applied, and also in individual lubricants. A suitable interpretation of the analysis results allows to warn of forthcoming machine damage symptoms and it also allows to identify the place of creation of the mechanical defect.

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