Method for determining the sliding-friction coefficient of the aluminum-zinc-pla

Products & Technique Management Department,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201999,China

Abstract: With reference to the testing standard ASTM D1894,the sliding-friction coefficient of the aluminum-zinc-plated post-treatment plate was measured.To reduce the detection error in practical applications,a correction factor is proposed.Although the test results showed that the relative standard deviation of the friction coefficient of the aluminum-zinc-plated post-treatment plate was 12.1%,indicating that the test results experienced a certain fluctuation,it can nevertheless be used as an internal factory-controlled test method.At the same time,problems were encountered with respect to sample contact,sample edge burr,and sample surface contamination.However,measures can be taken to reduce the impact of these factors.

Key words: sliding-friction coefficient; aluminum-zinc plate; post-treatment plate

1 Introduction

The aluminum-zinc substrate on the aluminum-zinc-plated post-treatment plate is coated with a layer of organic film,the thickness of which is usually about 1-3 μm[1-2].The main types of organic film used include passivation,fingerprint-resistant,self-lubri-cating,and highly weather-resistant films.The alumi-num-zinc-plated post-treatment plate can exhibit a “collapse” phenomenon during the packaging,transpor-tation,and storage processes,which is directly related to the friction coefficient of the organic film on the post-treatment plate.The smaller the friction coef-ficient,the more easily it can slip between the coils and the more likely it is to cause “collapse”.Con-versely,the greater the friction coefficient of the organic film,the less likely it is to cause “collapse”.To reduce the friction between the plate and the die in the stamping process,and to prevent the plate from cracking and other defects,the plate is coated with a layer of self-lubricating film[3].Therefore,it is necessary to control the friction coefficient of the aluminum-zinc plate.

The types of movement-related friction include rolling friction and sliding friction.Rolling friction occurs when an object rolls along a contact surface,whereas sliding friction occurs when two contact surfaces or points slide against each other.The sliding-friction coefficient is determined based on the mechanical properties,geometric properties,lubrication conditions of surface materials,surfaces,sizes,and other influencing factors[4].Measurement of the friction coefficient of the ball-pressure sliding friction between a ball head and a sample with point and surface contact has been widely used for the aluminum-zinc-plated post-treatment plate[5-6].How-ever,there are few reports of the measurement of the friction coefficient of the sliding-block friction between test surfaces.This paper mainly discusses the application of the sliding-friction coefficient of a sliding block to the post-treatment plate.

2 Test method

2.1 Selection of testing standard

Because the surface of the aluminum-zinc-plated post-treatment plate is coated with a layer of organic film,the test standard for its sliding-block sliding-friction coefficient can be considered with reference to either GB 10006/ISO 8295 “Plastics—Film and sheeting—Determination of the coefficients of friction” or ASTM D1894 “Standard test method for static and kinetic coefficients of friction of plastic film and sheeting”.However,GB 10006 is applicable to thin plates with thickness of less than 0.2 mm,and the aluminum-zinc-plated post-treat-ment plate samples are greater than 0.2 mm,so the ASTM D1894 standard was selected as the test standard for this test.As shown in Fig.1,one sample was cut into a small sample B and large sample C,and the non-test surface of small sample B was glued to slider A,which had a certain total mass.The large sample C was fixed onto test bed D with the test face upward.Under the action of tractionP,the small sample B and slider A slid forward.By recording the tractionP,the sliding-friction coefficient of the aluminum-zinc-plated post-treatment plate was calculated.The key technical par-ameters for testing the friction coefficient accord-ing to the ASTM D1894 standard are as follows:a small sample size of 63.5 mm×63.5 mm,a large sample size of 250 mm×130 mm,a test speed of 150 mm/min,a test stroke of 130 mm,and a total mass of the slider and small sample of (200±5) g.

Fig.1Schematicdiagramofthemeasurementmethodusedtodeterminethesliding-frictioncoefficient

2.2 Correction of test data

The test standard ASTM D1894 requires that the total mass of the slider and small sample be (200±5) g.Because the quantity of the small sample of aluminum-zinc plating is generally 10-40 g,strict-ly speaking,the determination of the sliding-friction coefficient of the aluminum-zinc-plated post-treatment plate does not meet the requirements for an ASTM D1894 test sample.As such,if the above standard method is directly used for testing,the error of the measured friction coefficient will be large.To reduce systematic error in the friction coefficient results,a correction factorfshould be used to modify the test results.

The formula for calculating the sliding-friction coefficient is shown in Equation (1):

μ0=F/Fp0

(1)

where,μ0is the friction coefficient,which can be read directly from the friction coefficient meter;Fis the friction;andFp0is the normal force.

The normal forceFp0in Equation (1) is the force applied vertically to the two contact surfaces of the sample,that is,the force exerted on the total mass of the slide and small sample.Because the small sample is light in weight,in the ASTM D1894 standard,the normal forceFp0is a fixed value,as shown in Equation (2):

Fp0=mg

(2)

where,mis the total mass of the slider and small sample,i.e.,200 g;andgis the acceleration of gravity.

Since the mass is larger in small sample B of the aluminum-zinc-plated post-treatment plate (ms),the forceFpapplied vertically on the contact surface of the sample is expressed as shown in Equation (3):

Fp=(200+ms)g

(3)

When considering the massmsof small sample B of the aluminum-zinc-plated post-treatment plate,μis the corrected friction coefficient of the aluminum-zinc-plated post-treatment plate,for which the calcul-ation formula is expressed as shown in Equation (4):

μ=F/Fp

(4)

By substituting Equations (1)-(3) into Equa-tion (4),Equation (5) can be obtained for calcul-ating the friction coefficient of the aluminum-zinc-plated post-treatment plate:

μ=fμ0

(5)

where,fis the correction factor,that is,f=200/(200+ms).

It can be seen from Equation (5) that the modi-fied friction coefficient is smaller than that directly determined by the ASTM D1894 standard.

3 Results and analysis

3.1 Stability of test results

We took one aluminum-zinc-plated post-treatment plate and cut it into 11 sets of test plates,each of which included one small and one large sample.The friction coefficient was measured according to the ASTM D1894 standard and then modified to determine the stability of the results,as shown in Table 1.As can be seen in Table 1,the relative standard deviation of the modified friction coef-ficient was 12.1%,which is slightly larger than the 10.0% normally required,indicating that the modified friction coefficient for the aluminum-zinc-plated post-treatment plate had a certain fluctuation.However,this value can still be used to monitor changes in the friction coefficient of the product.As an internal factory-controlled method,it has a practical application value.

3.2 Comparison of test results

The friction coefficients of five aluminum-zinc-plated plates of different post-treatment modes were measured using the sliding-block friction-coefficient instrument produced by TMI in the USA and the ball-pressure friction-coefficient instrument model 14FW produced by SHINTO in Japan.Fig.2 shows the two test instruments,and the measurement results are shown in Fig.3.As can be seen in Fig.3,although the measurement methods of the two friction-coef-ficient instruments were different,the trends of their measurement results were consistent and were related to the physical characteristics of the surface of the organic membrane.Therefore,the ASTM D1894 standard can be used to determine and modify the friction coefficient,and thereby serve as a method for evaluating the frictional properties of the aluminum-zinc-plated post-treatment plate.At the same time,it can be seen in Fig.3 that the friction coefficient measured by the sliding-block friction-coefficient instrument was slightly larger than that measured by the ball-pressure friction-coefficient instrument,which may be due to the sys-tematic error of the two methods.

Table 1 Stability of the friction coefficient

Fig.3Frictioncoefficientsmeasuredbysliding-blockandball-pressuremethods

3.3 Method limitation and improvement

The method of using anffactor to modify the friction coefficient,as proposed in this paper,is charac-terized by a certain fluctuation in its actual appli-cation to the aluminum-zinc-plated post-treatment plate.This fluctuation is related to the fluctuation of the sample itself and may also be related to the inadequacy of this method.

3.3.1 Insufficient contact between samples

The test object for this method is an aluminum-zinc-plated post-treatment plate,which differs substantially from plastic film.Under certain pressure,it is difficult for the sample to make full contact with the contact surface,so the effective contact area is smaller than the small sample area (63.5 mm×63.5 mm).This discrepancy has a certain impact on the test result[7].If there is an obvious radian on the aluminum-zinc-plated post-treatment plate,such that the plate cannot be leveled under the pressure of the sliding block,it is not suitable for this method.In this case,the friction coefficient can be measured using this method only after leveling.If the contact surface of the sample is uneven and not in full contact,the error of the friction coefficient measured using this method will be large.However,the difference in the friction coef-ficients between samples with similar concave and convex characteristics can be investigated using this method.

3.3.2 Small sample edge-burr effect

If there is an obvious surface burr on the sample,the contact area between the small and large samples will be the burr,which will make contact as a point on the surface rather than contact between the two surfaces as required by the ASTM D1894 standard.Therefore,the error in the detection results will be very large.Edge burrs can be removed with a file prior to measuring the friction coefficient.In addition,when the sample is sheared,the small sample should be processed into four edges with any burrs facing the non-detection surface.

3.3.3 Influence of sample surface contamination

The friction coefficient of a sample surface is related to its physical characteristics,so dust,finger-prints,grease,and other foreign substances on the sample surface will affect the test results.There-fore,prior to determining the friction coefficient of the aluminum-zinc-plated post-treatment plate,the sample surface must be gently wiped with anhy-drous ethanol to remove any foreign substances from its surface.Clean gloves must also be worn during the test to prevent fingerprints from manual handling of the sample.

4 Conclusions

According to the ASTM D1894 standard,the slid-ing-friction coefficient of an aluminum-zinc-plated sample was determined and the proposed correction factor was applied.The relative standard deviation of this corrected friction coefficient was 12.1%,which indicated that the detection result had a certain fluctuation.However,the trend of this method is consistent with that of the ball-pressure friction method.Both methods reflect the physical charac-teristics of the sample surface.The proposed method,which can be used as an internal factory-controlled method,can be applied to aluminum-zinc-plated post-treatment plates,and has a certain practical application value.